Telling us it freezes one-two seconds into a game, really doesnt tell us anything. DO you have to hard reboot or does it just crash? Do you get any beeping? What games are you running it in? What else are you running? 500W should be more than plenty to run a 7800GT. TO me it sounds like a driver problem. If it was the PSU, the card wouldnt be working at all. What are you +12V ratings?