To run PIIIs on the bp6 you need some PPGA-(FC-PPGA) converters. get more info from the forums at www.Bp6.com.
you will have to solder some cables, change voltages (yes the bp6 supports 1.6v cpu cores) Overclock the AGP bus to 89Mhz (133/2) and probly change one of the capacitors (EC10 between the cpus) to 2200uF 16v. will it run stable? probly not (got one myself! Cel400@552Mhz).
I think it would be better to buy a VP6 or MSI 694D or wait for some Dual/Quad socket A boards.