Here's a review of the nvidia 5600 & 5200 from Tom's hardware:
review
Looks like for the most bang for your buck to hit a radeon 9600.
Newegg
If I could recommend anything, I would recommend a Radeon 9700, as it is soooooo sweet. Unfortunately its not the cheapest at around $310.
I agree that the ATI drivers used to be pretty unstable, but from my experience in the past year, I have nothing but good things to say about them.