I built my system last year and have just been dealing with the issue by not over clocking after trying 3 different HD4850 cards. Current card in question is GIGABYTE GV-R485ZL-512H Radeon HD 4850 512MB
The current build is:
E8400 OC'd to 3.9ghz
GIGABYTE GA-EP35-DS3L LGA 775 Intel P35
OCZ GameXStream OCZ700GXSSLI 700W ATX12V
CORSAIR DOMINATOR 4x2GB TWIN2X4096-8500C5D
3 Sata HD's
The system runs flawlessly, however, anytime I attempt to OC the 4850 even by one notch the computer eventually crash after a short time period. I returned the first card I bought when this happened, tried the sapphire card brand had the same issue, and went again with this gigabyte card and to no avail had the same issue with being unable to OC the card what so ever. I have tried using different connections from the PSU and has had no effect. Anyone have any ideas? Only thing I can think is either a faulty PSU or Mobo... that or my computer just hates ATI!