Originally Posted by 3S00W
T1i ISO rating Auto, 100, 200, 400, 800, 1600, 3200 (6400-12800 with boost)
Someone please explain "with boost"
I'm not exactly sure the mechanics of how they do it, but the boost is basically artificially enhancing the ISO. It will lead to images with VERY high noise levels. In most instances you will never want to use it.
As far as noise levels, I have the older model, the XSi and images taken at ISO800 are very low noise and produce some very nice pictures. ISO1600 tends to be very grainy and I don't use it regularly. With the newer model, the noise reduction might be improved and could yield good results at ISO1600 but I couldn't tell you for sure.
Check out dpreview.com for sample images from different cameras to get an idea of what would be a good fit for you.