9600GT Rendering Problems

Ok, I just got a new PSU (OCZ StealthXStream 600W) and a XFX 9600GT for my PC. When I first installed the new hardware I had a few powering problems but after that, most of my games working, (Lost Planet Video test was around 50fps etc etc) but only a day after installing them, whenever I play a game (Most commonly on Guild Wars and Sims 2) the picture just plain glitches out. Texture go all scrambled but the game doesnt crash it just goes crazy. Any ideas for whats happening, I think its a power problem but Im not sure.

Extra Info: Windows Vista 32-bit I think, Intel Pentium Duo 2.80Ghz, 2GB of RAM. The card is brand new so I really need help.