Hi, I've just recently had my nvidia 8600gts driver crashing. Each time it would freeze then the screen would turn black and reappear saying "Nvidia kernel mode has stopped working and has been recovered" etc.... I tried numerous drivers but with no luck. I then tried a hd4650 but I get no better luck, although it does not crash as much.
Oh and the only changes I've made to the system is I reset the bios due to a unstable overclock.
I would power your pc down...make sure all the power and memory sticks are seated and the fans are moving. with two video cards with the same issue sounds liek it could bad stick of ram or power supply issue. i run memtest and hardware monitor and check your 12v output.