As the neither Intel P4 or AMD platforms will support SMP in the near future, your abvious choice will be to go with Intel´s PIII Coppermine or PIII Xeon.
Boards for 100MHz FSB, based on the BX/GX chipsets have been around for almost 2 years, the SuperMicro P6 DGE (IDE) or DGU (U2 SCSI) for PIII or the S2 variants for Xeons, are extremely reliable, stable boards w/ the 440GX, supporting up to 1GHz PIII or 700MHz Xeons, memory support goes up to 2 Gig.
If you are really in "big spender" mood you could go for the Intel 840 boards w/ dual RAMBUS channels and FSB 133, but performance gains are abysmal compared to the high cost of RAMBUS modules, again SuperMicro offers a nice board, the PIII DRx.
If you are looking for reasonably priced chipset choices for 133MHz FSB PIII or even Quad Xeons, there is currently no alternative to the ServerWorks (formerly Reliance) chipsets, although only the newest boards with the HE-SL chipset will offer AGP support. Again, SuperMicro has excellent offerings (370 DLx or the announced 370 DE6 w/ AGP), Asus offers a nice board w/ on-board ATI, CUR-DLS.
Tyan, Iwill or AMI would be additional choices, but I have little current experience there, past encounters have not been too favourable.