If you're likely to want to shoot video as well as stills you may be better off going for the new 650D if it's within your budget. My wife was using my 550D and getting some great photos out of it but decided she wanted an upgrade to the 650D when it was released. The ISO performance, articulating touch screen (which is excellent and very responsive), higher fps and the wireless flash setup make it a very tasty package.
With the 600D you get the articulating screen and wireless flash, but I don't think the ISO performance is any better than the 550D and the fps is the same too. The sensor and the processor are the same as the 550D too, whereas the 650D has a newly designed sensor and uses the Digic5 processor which gives a better buffer rate too.