I have the same specs just about (600w PSU for me) and I own a Radeon 5850. I'm extremely happy with it and would suggest it any day. Since you haven't posted a budget or anything, it's difficult to suggest something like that. What resolution do you plan on playing games? Which games?
Edit: Sorry, didn't realize you had two of these. Reply to this if you want, but the guy above knows his stuff.
Ask yourself what monitor size you want to use, then figure out the graphics card.
1280x1024 almost anything will do, 1920x1080 will require something like an HD 5800 series or GTX 285/295
Im about to get a 5800 series along with a bigger monitor. im doing 1280x1024 with a nvidia 9500GT, and it can handle most games on that res, although im getting a little framerate drops in certain maps on MW2.
I would wait until the end of march, rumors say that nvidia are going to release their excpected GF100 cards ( GTX300 or GTX 400? )
If they are priced too high im sure ATI will lower prices a bit and you will be able to grab a 5850/5870 for cheap.
Thats my plan