I'm using an ATI Radeon 9600 Pro 128MB, and I like it. I also happen to be using an AMD Athlon XP 2500+ Barton and a MSI Nforce2-based MB. Yes, there was some great pain involved in this setup, but with the 2.6.X kernels and the new 3.7.6 drivers from ATI, things are smooth. Read that, UT2004 is smooth at 1024x768 32bpp, with all eye candy. Really, any problems you might hit would be very well covered here, Rage3d, and nforcershq (if you have a Nforce too that is.)
Ether or. Both cards should give you decent proformance. My choice was based on liking the bang for the buck I've gotten out of ATI's cards. The lifespan on my Rage128's was really good. I'm hoping for the same from my Radeon 9600's
Also, I made my decision when the whole screen-capture fiasco was going on with Nvidia.
But hey, I use vertical sync as well. Hate seeing tearing. So think of that as you consider this advice