I'm still using PS 7, mostly because my machine lags on CS and CS2.
CS still has the same layers interface setup as 7, just has a few more filters and functions. CS2 has a slightly different interface setup for layers, and a couple new tools and TONS of extra teeny tiny features that you might not end up needing.
CS3 is totally made for photographer... no joke. They just improved almost everything that was made for photography or even added new features made for us. Affiliated CS3 programs also. CameraRaw for example ~
I found it a little faster than previous versions, I've got an old dual G4 (1.25ghz). What I really notice is that I need more ram... but only cause I need it for bigger files... I think any version needs tons of ram if you plan to use it for posters or high res illustrations.