I am enthusiastic (maybe to the point of being anal) about the quality of images my cameras and I produce. – Because they can hold timeless moments that are many times captured in one shot. I do not know what type of photography is your focus, or who your target audience is, but I will share from my experience...

Purchased a Nikon D40 as my first DSLR years ago, to replace my aged - although trustworthy - Nikon F2 35mm film SLR. Have since progressed through the D5000 and currently own a D3100 and D5100. The D40 camera came with the same 'kit' lens you own. I quickly discovered that lens was not 'fast' enough for my needs. So I used the Nikkor 50mm prime F-mount lens that was on my F2 for a while, until I discovered that the 50mm prime converts to a field of view at 75mm with the 1.5x crop in the smaller APS-c sensor found in a DX format Nikon.

I use a blend of camera glass (Nikkor, Sigma, and Tamron). Have had a measure of success with the use of Nikon DX and FX DSLR imaging systems, because I am a stickler for understanding the technical specifications of the systems, and then being able to capture and create what I want from that technical knowledge in the field.

Something I take the time to do is insure that a lens is compatible with my cameras. There is no doubt that my Nikon D5100 was created to satisfy user concerns previous owners of the D5000 observed. The design, functional and innovative modifications from the D5000 to the D5100 were done quite nicely, I must say.

Nikon and Nikon ‘purists’ say that Nikkor lenses produce the ‘best imaging quality’. I can challenge the ‘best imaging quality’ presented by Nikon and Nikkor ‘purists’ in the images I have produced by less expensive (but equitable quality) Sigma and Tamron lenses.

With the popularity of the small sensor APS-c format found in the D5100 and other recent design Nikon DX format cameras, the standard ‘kit’ lens is often supplied at a very attractive price. As mentioned earlier, I discovered the 18-55mm Nikkor ‘kit’ lens provided with my D40 camera was lacking in features and in image quality. I took a photographer’s prerogative to replace the Nikkor 18-55mm lens with the Sigma 18-50mmf/2.8-4.5 OS - this glass is my workhorse on the DX format cameras I own.

Along with the large aperture of F2.8, the Sigma 18-50mm has an almost 3 to 1 zoom range, this compact lens is great for the manual setting shots I take, as well as quick snap shots on the fly. Also, because the lens has an f/2.8 aperture, this lens is capable of ‘fast’ photography - meaning it can be used hand held effectively in lower light, helping to eliminate blur from photos with Sigma OS (Optical Stabilization – same as VR on a Nikkor). Of course use of a tripod for night captures is my preference.

And finally - the glass in most lenses is good quality, although the final shape of a lens and the mechanisms surrounding it will vary by manufacturer. My former D40 and D5000, as well as my current D3100 and D5100 do not have a auto focus motor, so I purchase lenses with a built in focus motor.

By the way, I recently added a Nikon D700 FX to my camera bag.

Here is a link that shows and describes my Nikon DSLR equipment…

http://www.facebook.com/media/set/?set=a.1691960134184.2093517.1093178780&type=3

Here is a link to some of my recent work...

http://www.flickr.com/photos/73877479@N00/sets/72157627582945437/show/

The prior responders pretty much have it. I would only add that you are limited somewhat with your tele because of its maximum aperture. And you mentioned portraiture of coming grandchild. You might want to consider an 85 mm (the perfect lens length for 35 mm portraiture, and get it in the largest lens diameter you can afford in a good lens -- as a 1.8, or at least a 2.0 or 2.8 (you don't have to get a Cannon EFL f 1.0) so that you will have the capability of shooting in low light -- where you are somewhat limited now, whether it be landscape work, or kids around the house shooting with natural light for the softer lighting without going nuts with an array of lights , filters, reflectors and such. Too, the larger lens will be very useful in extending your capability to shoot in the fading light of the sports work I think you mentioned -- and you can't postpone the match or game, you've got to be able to shoot it in whatever light is available, cloudy or later in the day. You can always up the speed or close down the lens, but you can only open it up to its maximum f-stop, whatever that is. For that reason I always buy each lens in the widest aperture possible, except for the variables, and you have one of those that will cover most of the world you want to capture. After an 85 mm you might want to go to a 14 or 20 mm (and you'll really be able to get super depth of field nature shots) in the widest aperture available -- also usable for landscape work. Then your 4th lens might be a big piece of glass in 400 or 500 (maybe f-4) for sports again if you really want to do that much of it. I use two identical bodies, one with a 70 - 200 and a 500 or 600 on the other, mounted on a good monopod with a short top-mounted bar on the top. With that rig I can sit at a corner of the soccer field and shoot at either end, and everywhere in between. And my fellow photogs don't complain about me having two or three monopods or a couple of tripods sprawled around in everyone's way. Like the other folks said, you just have to work your way into expanding your equipment according to how you are led to use it. Never-the-less, you can't go wrong with the widest aperture available in the lens of your choice. Too, I believe it best to have 2 or 3 excellent lenses rather than 4 or 5 mediocre ones. Amazingly lenses improve with age (improving in lessening chromatic or spherical aberration), but you probably won't notice it without some special testing. So just buy the best you can initially afford. You'll likely be happier with fewer but better and lower f-stop lenses than a bunch of limited lenses.
And if you don't want to splurge, perhaps you could secure a job shooting a local team or league for some offsetting bucks. I shot the Chicago Fire with the contract holder one year. Also shot a Mexican 73-team soccer league, and the majority of the players wanted shots of themselves in action. Although its not my sport, you could probably do that with a baseball league too.
You can also set up with the organizers of tournaments too to shoot team play all day long, especially if you have someone to help you process and collect for the photos buyers select. I've shot horse races in Germany, Italian and Arab motorcycle races in Tripoli, Libya, and rodeo bull, bronco, and pony riders on the national circuit and a bunch of people always want action shots of themselves. Go to a local Rod and Gun club and you'll find people who will want shots of themselves on the firing line or in the clubhouse with the trophies in the background. Again, if you have big glass you won't have to fool with a bunch of lighting paraphernalia. Everybody shoots landscapes, and it's interesting, but how many will you be competing against in sports photo work? Few, if you decide you want to do much of it. Catch your local high school or college football and soccer and bizball matches, and maybe do track meets. The only photog you'll see out there will be someone shooting strictly for the local newspaper, or some friend or parent with a point and shoot. Volleyball and basketball are possibilities too, but you probably have the lighting to worry about again with the relatively low light coupled with fast movement. Infinite interesting and challenging possibilities out there. Pick one or two and give it a whirl and pay for some additional "big glass" as you evolve your direction and your equipment. Have fun !

I am enthusiastic (maybe to the point of being anal) about the quality of images my cameras and I produce. – Because they can hold timeless moments that are many times captured in one shot. I do not know what type of photography is your focus, or who your target audience is, but I will share from my experience...

Purchased a Nikon D40 as my first DSLR years ago, to replace my aged - although trustworthy - Nikon F2 35mm film SLR. Have since progressed through the D5000 and currently own a D3100 and D5100. The D40 camera came with the same 'kit' lens you own. I quickly discovered that lens was not 'fast' enough for my needs. So I used the Nikkor 50mm prime F-mount lens that was on my F2 for a while, until I discovered that the 50mm prime converts to a field of view at 75mm with the 1.5x crop in the smaller APS-c sensor found in a DX format Nikon.

I use a blend of camera glass (Nikkor, Sigma, and Tamron). Have had a measure of success with the use of Nikon DX and FX DSLR imaging systems, because I am a stickler for understanding the technical specifications of the systems, and then being able to capture and create what I want from that technical knowledge in the field.

Something I take the time to do is insure that a lens is compatible with my cameras. There is no doubt that my Nikon D5100 was created to satisfy user concerns previous owners of the D5000 observed. The design, functional and innovative modifications from the D5000 to the D5100 were done quite nicely, I must say.

Nikon and Nikon ‘purists’ say that Nikkor lenses produce the ‘best imaging quality’. I can challenge the ‘best imaging quality’ presented by Nikon and Nikkor ‘purists’ in the images I have produced by less expensive (but equitable quality) Sigma and Tamron lenses.

With the popularity of the small sensor APS-c format found in the D5100 and other recent design Nikon DX format cameras, the standard ‘kit’ lens is often supplied at a very attractive price. As mentioned earlier, I discovered the 18-55mm Nikkor ‘kit’ lens provided with my D40 camera was lacking in features and in image quality. I took a photographer’s prerogative to replace the Nikkor 18-55mm lens with the Sigma 18-50mmf/2.8-4.5 OS - this glass is my workhorse on the DX format cameras I own.

Along with the large aperture of F2.8, the Sigma 18-50mm has an almost 3 to 1 zoom range, this compact lens is great for the manual setting shots I take, as well as quick snap shots on the fly. Also, because the lens has an f/2.8 aperture, this lens is capable of ‘fast’ photography - meaning it can be used hand held effectively in lower light, helping to eliminate blur from photos with Sigma OS (Optical Stabilization – same as VR on a Nikkor). Of course use of a tripod for night captures is my preference.

And finally - the glass in most lenses is good quality, although the final shape of a lens and the mechanisms surrounding it will vary by manufacturer. My former D40 and D5000, as well as my current D3100 and D5100 do not have a auto focus motor, so I purchase lenses with a built in focus motor.

By the way, I recently added a Nikon D700 FX to my camera bag.

Here is a link that shows and describes my Nikon DSLR equipment…

http://www.facebook.com/media/set/?set=a.1691960134184.2093517.1093178780&type=3

Here is a link to some of my recent work...

http://www.flickr.com/photos/73877479@N00/sets/72157627582945437/show/