Again, be sure to perform the deactivation AND trash old Photoshop preferences before reinstalling Photoshop clean. Which versions of Photoshop will be able to work on your Power PC (PPC) or Intel Mac will depend on a combination of your operating system, processor, RAM, video card and Adobe Creative Suite.
CS5 on OSX is also 64-bit (you can work on larger file sizes, helpful for LF scans). On Windows, CS4 is 64-bit but of course doesn't have content-aware fill. CS3 was so long ago I don't even remember what was on it.
Been working merrily away in ignorant bliss with Adobe's Creative Suite 3 since it came out and have not paid any attention to Photoshop's evolution.
But, I'm contemplating a new computer, which prompts the software upgrade question.
If you've done all the troubleshooting and reached the point of un-installing Photoshop to re-install, Archive install, clean install, or Erase your hard drive to "nuke" problems like force quit required, won't open and program errors, be sure to first Deactivate Photoshop so you don't waste one of your limited Activations when you re-Activate.
Deactivate before uninstalling or you may need to contact Adobe customer service to explain the problem and perform a manual telephone activation.