I found an easier way to OC in crossfire mode, if you run dual monitors. I have one monitor hooked up to the primary card, and the second monitor hooked up to the secondary card.

Disable crossfire.
Go into display settings and extend desktop to the 2nd monitor.
Launch rivatuner or whatever apps you use to OC. (I have only tried rivatuner)
Select your cards, and overclock them. Shouldn't matter which card you choose first.
Then enable crossfire. (this will disable the second monitor)

This still may work if you have one monitor. But you'll have to extend it to the second card. (haven’t tried)

I think i've reached about max potential with my mobo/psu. I was able to overclock in xfire mode to 820/970. Any higher and I got mixed results, I either couldn't enable crossfire mode, or the machine would crash/reboot.

So I hooked up a spare psu laying around. I hooked them up to the 6pin plug. I didn't get much better oc's as I still suspect the power supply still isn't enough. I was able to get to 829/990 with the extra psu.

I also was unable to overclock in xfire mode in Vista. but was fine in XP.

at 829/990 I was only able to score ~16110. This is where I think the p5b and a c2d reaches it limits. At stock XT speeds, I got 15131. With a 78mhz oc on the core and 142mhz on the memory, I got about 1000 points more. With the x4 pci-e handicap, I should be happy with these results that it was able to even scale this high. Not as high as I wanted, but that’s because of the mobo/cpu.

definitely a slippery slope. I upgraded the cards, now I want to upgrade to a X38 and a more powerful psu.