Well, I recently purchased an 8800gt card to replace my old x1950pro card. All of my system components are new, only 3 months old. Only problem is, after about 30 minutes of graphic intensive games like Call of Duty 4, my computer freezes. Everything seems to be in working order, and have had no previous issues. Im running on a Corsair 450vx 450w psu, with 33a across a single +12v rail, which should be ample enough. All of my drivers are updated, everything is connected, but still, the random freezing during gaming is a pain. P.S. this seem a little bit off, but Speedfans and Everest voltage readings are only picking up 2.43v on the +12v rail. obviously this seems to be an issue, but I'm not quite convinced considering I was able to play CoD4 easily on the power hungry x1950pro. Anywho, thanks in advance, I hope ya'll can figure this out.

Your computer would not run if you were only getting 2.43v on the +12v rail.
Did you uninstall the old video card drivers?
What temps are you running? ( Video, CPU, Case...)