I have two computers running F@H smp.
#1 = C2D 6550, 3.2GHz, 4GB ram, Gigabyte P35 motherboard. This is running XP 64 bit.
#2 = C2D 6420, 3.6GHz, 4GB ram, Gigabyte P35 motherboard. This is running Vista 64 bit.
#2 produces half the WUs as #1. I wouldn't believe the difference in CPUs would...