What CPU clock speed should I need for Crossfire?

[BangGlorious]

Hi, recently I upgraded to have 2 Radeon 6850's with a Crossfire set up (Both are overclocked to 900MHz). Liking improvements to my frames so far but it seems I am being bottle-necked somewhat by my CPU (AMD Phenom II X4 955) which due to a little mishap with a pin I can't overclock it too much without running into a power strain issue leading to a bsod. It seems some games react differently (for instance BF3 I could go up to 3.6GHz on my CPU and be stabled while with Tribes Ascend it doesn't seem to like anything over the standard 3.2GHz clock). Anyway I've been thinking about finally upgrading my CPU anyway since I've had it for a while and was wondering what the minimum clock/bus speed would be so that my 6850's are not bottle-necked. (Also any good suggestions for a CPU? I like ones that are good at overclocking-preferably above 4.0)

Additional Info:
ASRock 970 Extreme4
Windows 7 64bit
8GB DDR3 1600