Freezing problems with OC and overvoltage of Memory

I have the ASUS p5n32-e sli 680 with the 1002 bios and e6600 week 44G with OCZ gold pc-6400 memory. Well I think I am having compatability problems with my memory. Every time I try to overvolt it to say 2.1 or anything above the stock, my computer freezes shortly after I log onto windows. I ran memtest for 6 hrs and it came up with out any errors, does anyone else know what the problem could be?