I just read three pages of very well-intentioned responses to your question, but none have asked the most important question - what software do you use?

You need to know how the software engineers have written their software before you can decide on a configuration. I think you'd want a machine that you can use today, and still provide some expansion capability for the future. Without knowing the specifics, you are using a D800. These generate 45mb raw files, and if you edit a file in Photoshop, in 16 bit, you will end up with an image that is typically 500mb, or more if you make extensive use of layers. If you use content aware features, gaussian blur, stitch together panoramas, do focus stacking and HDR, etc - 16 gb ram is not going to be enough. With 32 gb ram, you will be able to edit .psb files, which are 2 gb or bigger. I do it all the time with 32 gb. If you use Photoshop, b\uying a computer with 32 would be the right move for your needs today, and in the near future. Having the capability to expand to 64gb would give you a degree of future proofing. As we have all experienced, ram is constantly getting denser and cheaper. Buying more than you need today may not be the best use of your $$$. If you wait until you actually need it, you may end up paying a lot less. Just make sure your motherboard has enough slots, and that your current configuration leaves enough room for adding memory without tossing lower density chips. When the time comes, you want to be able to buy just 32 gb ram, and not 65 gb, tossing out your 8gb sticks and replacing them with 16 gb ones.

Lightroom, if you use that, is a horse of a different color. It does not improve appreciably with more than 16 gb. When I went from 16 to 32 gb, I really didn't notice any improvement in performance. But LR did not have HDR or pano stitching. What LR does utilitize is mutliple CPU cores - 4 is considered minimum, and it will use up to 6 or 8 if you have them. There are 6 and 8 core i7 cpus (68XXa series) that really fly with LR.

Graphics are even easier. There is no real benefit to having more than 4 gb vram unless your use includes driving a pair of 5k displays with a 4k video editing application. The extra ram is used for frame buffering for video at high frame rates - mostly for a smooth gaming experience or for 4k video editing.

Howeveri f you have a 10 bit capable display and you want to be able to turn on the 10 bit graphics capability in Photoshop, you will need a 10 bit card - your choices are either NVidia Quadro, or ATI Fire Pro cards. The card needs to support the Open GL and Open CL libraries. That's it. The newer non-10 bit cards are slightly faster when used with LR or PS, but just barely. A 5 yr old NVidia GT550 with 2 gb ram will easily drive a pair of 1920x1200 displays with decent performance. I went from that card to an NVidia K2200 with 4 gb, and the performance improvement was modest at best. I have one student that has is a gamer and uses Photoshop. He did not see any difference upgrading to a GTX1080 with 8 gb from a GTX 960 with 2 gb as far as PH was concerned. He did see a substantial improvement with his gaming apps though. Fast hard drives are considerably better than fast graphics cards loaded with lots of vram for Photoshop. And LR is even less dependent on GPU speed and vram.

Here are some guidelines from Adobe on graphics cards:

https://helpx.adobe.com/camera-raw/system-requirements.html#General
https://helpx.adobe.com/photoshop/kb/photoshop-cc-gpu-card-faq.html
https://helpx.adobe.com/photoshop/kb/photoshop-cc-gpu-card-faq.html#CardRequirements

I'm buying a new computer before Christmas. I've decided upon the make, model, etc, etc. Except for one thing. There are two versions of this computer. One has 24gb of ram and the other has 64. It costs $300 more to get the 64gb model. This computer will be used for image processing, web surfing, emails, and that's about it. No gaming. Do I need the extra ram and, more importantly, will it make a considerable difference in image processing? My cameras are Nikons D3, D800, and DF, if it matters.

This is the correct answer - you answered it yourself! Dell does not manufacture computers for photo processing. They make them for business environments, gamers, and families. Moreover, Dell computers as well as several other name brands (Compaq, HP, and more) use many proprietary components, so you might be limited if you want to upgrade or replace a failed part.

I would recommend you decide what you want then either build it yourself (it's not terribly difficult) or find a friend or reputable shop to build it for you. Adobe.com even has recommendations.

This is the correct answer - you answered it yourself! Dell does not manufacture computers for photo processing. They make them for business environments, gamers, and families. Moreover, Dell computers as well as several other name brands (Compaq, HP, and more) use many proprietary components, so you might be limited if you want to upgrade or replace a failed part.

I would recommend you decide what you want then either build it yourself (it's not terribly difficult) or find a friend or reputable shop to build it for you. Adobe.com even has recommendations.

I'm buying a new computer before Christmas. I've decided upon the make, model, etc, etc. Except for one thing. There are two versions of this computer. One has 24gb of ram and the other has 64. It costs $300 more to get the 64gb model. This computer will be used for image processing, web surfing, emails, and that's about it. No gaming. Do I need the extra ram and, more importantly, will it make a considerable difference in image processing? My cameras are Nikons D3, D800, and DF, if it matters.