Well that's a pretty steep price to pay for something you are unsure if you need all of its capabilities. I actually prefer the pre-CS versions of Photoshop. Maybe you can find Photoshop 7 or so for cheaper, since it still has tons of features, but they might not sell it anymore. What exactly are you looking to do?
paint.NET is free and great for simple tasks but probably not as powerful as PSP 8 even. Gimp is free and quite similar to Photoshop, but I've never gotten the hang of it. I guess I"m too used to the Photoshop UI and shortcuts that I always get disgusted with Gimp when I use it. But that would be something you could download and try out before dropping down the $ on Photoshop.
Finally, PS Elements is a stripped-down version of Photoshop. It may or may not have all of the features you need, but there may be a trial version for Elements. After checking that out, you can decide if it'll work well enough for your needs. It's like $80 for v4 and $130 for v5 (includes Premiere, so try and find it by itself for cheaper).
It's probably worth the few hours to check out the alternatives before spending $650 on PS CS2, since depending on your skill level and features you require, the cheap/free alternatives or older versions may be plenty adequate.
edit: Yeah, you can find Photoshop for cheaper. Newegg has CS2 full for $570. Newegg also has a CS2 trial for $6, so there might be a downloadable trial for free. And Newegg is the only place I looked, so it might be slightly cheaper elsewhere.