The main difference between 7.0 and CS was a few VERY useful color matching tools which I never really learned to use properly but I'm working on that.

The main addition 7.0 had was the healing brush -- sort of like the clone brush but very usefil for fixing things like red-eye and scratches on photographs. More for hard-core photoediting than graphix work.

6.0 is the earliest back I would go and still call the version a good one. Personally, I use 7.0 at the moment.