I would recommend AMD as well, as their 64 bit CPU's are very reliable right now. I currently have an AMD Athlon 64 3200+ (socket 754) running on an Asus K8V SE Deluxe, and it runs very smoothly. (I may upgrade to socket 939 in the future)
X800 is better for dirext X and Half Life 2, but the 6800 has Pixel Shader 2.0, 16 pipelines and some other nice features. It all comes down to whether you like Nvidia or ATI.