I'm having a ton of trouble myself with my new i5 750 and 4x2gb dimms.

I can do 3.5ghz @ 1.3v core and 1.15v VTT, all else stock.

3.8ghz@ 1.3875v core and a monstrous 1.41v vtt. I'm sure the insane VTT is because of the 4 dimms being used :/