Either a Pentium 4 or an Athlon 64 will work, with the help of this sister board called the A9S, but you can't run both at the same time.

A9S is the name given to the sister board for the Athlon 64, which plugs into the Elite Bus Slot on the PF88 Extreme motherboard. At present the A9S is the only such sister board available from Elitegroup. But other combinations are conceivable, as long as they work with the Southbridge SiS965 and they're compatible with the SIS MuTIOL protocol (used to connect the Northbridge and Southbridge chipsets in the SIS HyperStreaming architecture).

We observed an interesting phenomenon during testing: our Windows installation couldn't adapt when we witched from a Pentium 4 to an Athlon 64. All our recovery efforts were in vain: no matter what we tried, Windows XP experienced a total system crash. But going the other way was no problem: after a clean re-install of Windows XP we could transform an Athlon 64 system back into a Pentium 4 configuration without a hitch. We also observed that the system switched over without any errors, and also that it showed no reductions in performance or other weaknesses after the switch was complete.

The SiS756 is on the right, equipped with a heat sink to handle the heat its hefty power consumption generates.