BobInNJ Loc: In NJ, near Phila. Pa

First let me say this site is awesome. As someone tying to graduate to "photographer" I am taking more than contributing. I hope to be able to improve and help someone else!
OK- In an attempt to brighten up interior shots of homes in my Real Eastate business, I usually set ISO to 1600 (max. aailable on my Sony Alpha A-100). I just read a piece here in UH that said start at 200 ISO and go up until you get good results. I was stunned!
My goal is to capture interior without flash, as flash washes out so much. ( I invested in Sony's $200 flash attachment , as on board flash not sufficient for larger rooms- I probaly need to tone it down) )- So, do I learn how to use the flash better, or, learn the correct ISO settings? Thank you for reading all this- I owe you one!
(example of mediocre results attached for comments)

fcrawley Loc: Melbourne, FL

You can do both, depending on the natural light entering the room. Using a tripod will really help using natural light without a flash. If you do that, make sure to set your white balance for the natural lighting. When using a flash, experiment with bouncing it off different surfaces, not just direct. You also may want to use a circular polarizer which will cut the glare from the glass items in the picture (windows, table), but it may not work on both at the same time. With TTL on your camera and the dedicated flash, you can also set it up for fill flash. Read the two manuals on how to adjust your settings for the flash on your camera. Then pic a room, such as the one you showed us, and try a lot of this out to see what you get. Since you are shooting digital you can really experiment, but one big big hint, TAKE NOTES. Write down what you do for each shot as you are experimenting so that you can remember what you changed in each shot so you can learn how the changes affect the image. This way you can learn about different techniques and use them in different combinations under different conditions. You also may want to invest in some fluorescent continuous lights if you have the time to set up and use them when you go into a home.

CamObs Loc: South America (Texas)

Just a suggestion on Arch shots. You probably use a very wide angle lens. Make sure it is level on the tripod (bubble levels are cheap) and in PP make sure the vertical surfaces are vertical (check perspective box in PS and move the frame). If you need flash, use a bare bulb. You have done a decent job (on the one included above) by exposing the outside and the inside of the property.

RoysJungle Loc: Ohio

I seen on one of the episodes of D-Town where RC was shooting for selling a house for sale and used HDR imagery to get could pictures in low light inside the house maybe you could try this along with using a tripod so you can use lower ISO and longer shutter speeds. Just if you over do the HDR blending it will start looking cartoonish or surreal.