I had the same problem running my Nvidia 7800GTX, freezing then randomly reboots

I bought a 500w PSU and it still did it, then i bought a better 550w PSU for £65 and it still does it, i then proceeded to do more tests on the PC to find my 2x RAM sticks were running different speeds and clashing, so bought 2x 1GB matched Corsair platinum sticks (£150) and it still does it

I have since tonight bought extra cooling for the CPU and GFX card, bought a big mofo arctic cooler and an exhaust fan, upto ive had no crashes but we will see!