drjohn, VIA is expected to deliver PCI Express for Athlon 64 in Q2. DDRII, who knows, can be in one of the next steppings already. AMD said the support is trivial.
Socket 939s cpus should have DDR500 support and PCI express m/b's which should comfortably do until DDRII667 is widespread. DDRII533 will be ignored by AMD as they have already publically stated many times.