depends on what upgradability you want.
Get a socket754 for cheap if you don't mind replacing the mobo next time you upgrade. they're really cheap right now because the socket has no future.
If you want to be able to upgrade it, go with the s939 Asrock mobo that newegg has for like $60, then blow the rest of your wad on your cpu and/or video card. The asrock will take an AGP or a PCI-e, and there's good deals to be had on solid AGP cards in the ForSale forum since everyone's going to PCI-e. Also it will apparently be upgradable to the M2 socket due out this winter for the next gen of AMD procs.
It's a good time to be building a cheapo workstation since so many people are migrating to s939/PCI-e right now.