I'd suggest something in a dual Opteron. Say, a Tyan K8WE, with support for two dual core CPUs, PCI-X and PCI-e, and as much as 16GB RAM.
That ought to cover all your bases for I/O and CPU horsepower.
Back in the land of reality, Gigabyte and Soltek make great Athlon64 boards.
At this point I...