Personally I wouldn't want to use anything below the xxd line - at the moment 60d - because it has a top lcd screen and, even more important, the back dial for exposure compensation et cetera. Once you get a bit more advanced, you won't want to miss these because you can get to the right setting much quicker. You can read about the other differences to the xxxd line elsewhere, ask if you want to know if a specific feature is important.
I second this! The 60D has the things you need over the 600D to make using your camera a significantly more enjoyable experience. The better grip, rear control dial, top LCD and excellent battery life will make a big difference. I have the 60D and can thoroughly recommend it if you want a Canon APS-C DSLR.