Any version from 7.0 on up is good. CS2 is 9.0 (if you watch the dialog that comes up as the program loads, it actually refers to itself as PS 9.0), and that's the version I use at home. I generally use CS3 at work, and while it has some advantages it's also kinda quirky and some of my partners don't care for it.
I think CS2 probably combines the best advantages of the new system (it does have a superior transform tool to 7.0, and I use that tool a LOT). It really depends on what you want to do with it... for painting, 7.0 is perfectly fine and I know several excellent digital painters who haven't bothered to upgrade past that point at all.