What he(^) said; additionally, you can find Gateway/ALR motherboards(7200 series) that are dual slot1 with (1) 64bit PCI slot, (5) 32bit PCI slots, and (1) PCI/shared slot(no AGP slot). Also onboard is dual-scsi(wide), video and 10/100 lan. You'll need a VRM for the 2nd CPU and the board is fairly large.. won't fit into all ATX cases.
How do I know this? I have one here(dual p3-500) that's crunching SETI units for our TeAm.