Why am i getting BSOD in-game?

Just installed windows 7 and added 4g of ram, everything seemed fine, but whenever i play crysis or bad company 2 or APB the comp crashes, i turned off auto restart so i could see the bsod, what i got was a STOP 0x0000007a error, ive been looking everywhere for answers, been cleaning registry, been checking for hard drive errors but i still dont know whats causing them (note: usually after crashing the games do run for a while but ive never witnessed them NOT crashing)