Well, the nForce2 is socket A, the K8M800 is socket 754. I've built many computers with both, and they both do 2D very well. Avoid SiS, their socket A stuff had terrible driver support, and I don't care to test out their socket 754 stuff anytime soon.
For building a new computer there's no reason not to get a socket 754 since the cheap Semprons were recently released, so I'd definately recommend the K8M800.