PC crashes while gaming

[LordBubbles]

Heya everyone.
To start with, 1st post, yay!

Now on to the problem.
About 2 months ago I started having a problem while gaming. I could play any games before that, with no problems.
My computer will freeze up, colours go funky, and I can't exit out, besides resetting. It will only happen on "high-end" games, namely Left 4 Dead, Guild Wars, WoW, and surprisingly it also happens on Google Earth.
Yet, I can play games like, Red Alert 2, and Age of Empires 2...

CPU: Intel Core 2 Quad Q6600 @ 2.4 GHz
Motherboard: Gigabyte S3 GA-EP35-DS3P
Ram: 3GB DDR2 (3 x 1GB)
GPU: NVidia GeForce 8800 GT
PSU: Not sure exactly, but i think its a CODEGEN 625W ATX 12V (at least thats what the label says.... =P
OS: Windows XP SP3

At first I thought it was overheating, then I thought it was the Power supply....
Can anyone help me??

(and yes I know I don't need that extra 1GB of RAM on XP.... slowly upgrading the computer, piece by piece)

 

[menacer]

Firstly make sure you have the latest nvidia driver... 182.08 at time of writing this.
Does your power supply have a dedicated pci-e feed or are you using the 2 molex to pci-e adapter?
If you are using the molex adapter try connecting them to different feeds from the psu. Low power getting to the card can cause your symptoms...