either the tyan tiger (mpx) or the asus a7m266-d (mpx)
both are based on the amd 762 mpx chipset, and both are very nice boards... the tyan is probably a little nicer (plus it costs a little more )
there is also a tyan tiger (mp), but its a little older and the (mp) chipset doesnt have the 64bit/66mhz pci slots like the mpx
or if you want a high end server board you can go with the tyan thunder (it comes with onboard u160 scsi, and is designed to fit in a u1 server case) but its quite expensive, costs over $400