Reseat the video card. Unistall all Nvidia drives and completely clean the registry. Reinstall the lastest Nvidia drivers and test your system......if that fails......then we'll have to loot at PSU, Run memtest or windows memory test and start swtiching parts aoround in another computer to find the culprit part or parts. Just because you have a 550wt PSU doesn't mean that the 12v rail voltage isn't all over the place after installing that video card. Try the driver issue first.
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.