With only one stick of ram installed update your vista to date, there is a patch in there that fixes issues with ram over 4 gigs, could be your problem.
If that doesn't work:
I'm assuming your not overclocking your CPU nor your ram atm; if you are, set to stock values.
try using different ramslots, meby one of those is bunk
or you could try Downloading riva tuner and set your vid card to stock 260 values, your video card is factory overclocked, but that doesnt mean its completely stable. (i dont believe its the vid card, but we need to remove all doubt)
give that a go bro, sorry we didnt help you sooner, christmas an all that eh?
__________________
vista 64 on these rigs
Q9450 @ 3.5Ghz | E8400 @ 4.2Ghz
8gb Muskin Redline. | 2gb Kingston Hyperx
________________________________|_________________ ______________
EVGA 780i FTW ----\--||___||--/---- EVGA 780i SLI
Antec TP Quatro 1000W \ :==^==: / Antec TP Quatro 850W.
Nvidia 9800 GX2 X 2 SLI \|--o--|/ Nvidia 8800 GTS X 2 SLI
1X 500GB Western Digital. \_____/ 3X 150GB WD RAPTOR RAID-0
|