Green screen?

[Mazza]

Okay when i tried to auto oc my GPU with ATi overdrive, at bout 850 from 750 i gets a green screen, i rebooted, green screen, booted up in vga mode (480X200 res 16bit colour) it worked googled abit, it said that a green or black screen may be displayed when going from something to something, wasn't that. Anyways did a re-install od drivers worked. My question is why would i get a green screen? I might be my PSU, only like 450W with a 20pin MB will it needs a 24pin  ;D