I've used the K8T800 so far, but it seems right now nvidia is pushing more performance out of their chipsets, so testing a setup with NF3-Ultra seems a good idea. My motherboard choice is probably leaning towards the MSI K8N series due to a good reliability track record from MSI in the past.
Beware of the MSI K8 boards. Apparently they had a bad batch of their K8T Master2 dual opteron boards - hundreds (maybe thousands) of people had unresolvable issues with them, including myself. I ended up getting a K8 board from Tyan.