I have been mining a single 5870, and now I have decided to go dual 5870s.

I have a certified crossfire 750 watt power supply

Motherboard with 2 PCI Express x16 slots

I expected to get 2x the performance 325 Mhash/s x 2 = 750 Mhash/s ( aprox )

But it turns out my rate was only 425 Mhash/s. Load was running around 5, so I am assuming my CPU is too slow to push both cards to the expected rate of 750 Mhash/s.

What is the minimum cpu required to get 750 Mhash/s from dual 5870 using crossfire ?

What's your miner? CGminer or GUIMiner? The CPU is kind of out the equation here, as I've successfully run 5970's with a paltry AMD Sempron single core.