I'd check the readme info for CS-3 first. I do know that if you had the Trial version of CS3 installed, Adobe forces you to un-install it first, as they don't seem to have a way of successfully installing the full version on top of it. Otherwise, the new version should give you an error if it won't work.
in my opinion, once you get used to CS3, unless your machine is over 2yrs old, and under-powered, CS3 will keep you from going back to PS7.
Good luck!