not necessarilt a quad core, fast dual core will be ok, like e8600. But yes, that CPU will hold back even a single 8800gt, let alone 2 in sli. If your into overclocking you could get that cpu to around 3ghz with voltage increase, so long as your motherboard can support higher FSB speeds. You should be able to get to around 2.7 ghz without increasing voltage. You should see some good gains in FPS from that. But a newer, faster e8600 will be better as it has more cache to go along with higher clock speed.
Thx for the advice, what software software should I use to overclock the cpu? Or can I do it in Bios ? I have a asus P5N-D(NVIDIA nForce 750i SLI)