It depends on your needs and your system. I had stability issues with CS3 and like CS4, but haven't tried CS5 (or CS5.5). I've talked with others who've had exactly the opposite results (CS3 stable, CS4 unstable).
Really the only limitation is on CS5 and it requiring a 64-bit system and codecs. If you have access to the various versions, try them all out and see which one you like best.
In your Control Panel, open up the System option. In that window, look for what your processor and RAM are.
On Windows XP, it'll look like this:
On Windows 7, it'll look like this:
The red boxes are around the important bits of information for your reference, they're not actually there when you open it up. I don't have a screenshot for Vista, but it'll be similar to the one for 7.