New
#11
Isn't the obvious thing to stick your old graphic card back in. Then test for however long. No probs should mean CPU is ok.
Then tackle new card - well known there can be incompatabilities with graphic cards and motherboards prob needs a bios tweek by the manufacturer
You went from 256 card to 1gb could be a memory overlap problem somewhere also.
Stick 1gb card in again.
Drastic - but you could delete "pci bus" driver in Device Manager then reboot and let win 7 rediscover all the drivers keep rebooting then go onto windows online update to find any others it needs and let them settle in. reboot.
See if it sorts it.