Upload & Sell: Off
1) Your T1i will be fine at ISO 400, probably even 800. Your final output is small size jpegs viewed on a monitor or mobile phone. The noise will be completely insignificant as long as you expose correctly.
2) You can get the image bright enough by just using a longer shutter speed. It is better to expose properly from the start than to boost in post-processing. This may be why you are seeing noise if you are underexposing and then fixing in post.
3) Always turn on all lights in the room. You are trying to balance the interior light with exterior. Exterior will be way brighter, so do as much as you can on the interior.
4) Ditch the Fong. Bounce your flash off the wall behind you. This is usually the best, but it will cause problems if the wall has a strong color. You can also point up, or up and in, but that may give you shadow problems with lamps, ceiling fans, etc, and/or a bright spot on the ceiling. Fixable in PS, but that adds a lot of work.
5) The flash should be in manual. If you are only using one light you, in large rooms you will often be in the 1/4 to full power range, especially when bouncing.
6) Use your tripod, start at an aperture of about f/5.6, ISO 400, and then either in M or Av (with exposure compensation) get a proper exposure, whatever shutter speed that is. (Note that this may be brighter than what the camera thinks is correct, since it will be thrown off by the bright windows.) Your windows will be very bright, possibly nuclear depending on time of day, size of room, etc.
7) How you like your windows is a bit of a style issue. I go for a more realistic look, which is also what you see in Architectural Digest, Pottery Barn catalog, etc. That means bright windows, but not nuclear. If you don't like the windows from your shot in step 6, shoot another one where the windows are exposed how you want them and the room is dark. May be down 3 stops, who knows. Then blend the two in PS.
8) If you want to do this well, and quickly, and get it right in camera to minimize post work, you need multiple flashes. You basically have two options. Option A is to go the pocket wizard route. This is expensive, but gives you lots of flexibility. Option B is to get off-camera flashes with built in slaves. Some old Nikon flashes have this (SB-26 or 28? SB-80dx? Promaster 7000M? Can't remember for sure.) Just use your Canon flash on camera in manual to trigger, put the off-camera flashes on stands and bounce them off walls which are not in the frame.
9) You can also do this with HDR or exposure fusion. Less time on site and less equipment cost, more time in post. Your on-camera flash will still be useful.
10) Visit the Photography For Real Estate group on Flickr to learn more about all this stuff I mentioned.