ATI and Nvidia are about equal when it comes to Direct3D performance. Nvidia though performs 2-3 times better in OpenGL games like Doom 3 and Quake 4. ATI cards can barely even play Doom 3, and I would imagine it ill be the same for Quake 4. Take it from me, I bought an X800Pro when it first came out, and those who regularly visit this forum know from my ranting that it just plain sucks.
Also, Nvidia features Shader Model 3 which is used to enhanced games like Battlefield 2 and is REQUIRED for Call of Duty 2 (otherwise it will run in DX7 forced mode with no shaders). Nvidia all the way....except for maybe the X1800XT, but that card is WAY overpriced right now.