Any of them will work. I wouldn't get anything less than CS3. I believe CS5 is the newest and it has some pretty nifty features.
If you want to go the free route you can try the Gimp. Its nowhere near as powerful as Photoshop but its easier to use if you're new to editing. Its absolutely free to use which is nice.
GIMP - Free software downloads and software reviews - CNET Download.com
I use Photoshop CS3 but I'm a bit rusty so if you see any works I do don't hold it against me. :P