Green dots then system lockup during 3d games.

Hi all, i hope someone can help.
My system has recently (last month or so) started locking up whenever i play 3d games (world of warcraft, call of duty: mw1 & 2 etc).

My PC specs are below: I bought it from overclockers.co.uk about 2 years ago now.

Asus M2N-SLI-Deluxe mother board.
AMD athlon 64 X2 5600+ (2.8ghz)
Geforce 8800GT
2GB RAM
Antec 550W PSU
SB X-Fi Xtreme
Antec 900 gaming case
Vista 32bit.

Now after reading many websites, i assumed it could be power related, so i bought a new PSU....
I got a Thermaltake w0093 off amazon and thought all would be fixed.
The problem still persisted, so i thought.. maybe its a graphics card issue... I bought a 9800GT, again... same issues.... still locks up whenever i try and play games.

By now i was lost for ideas, i tried both different RAM slots 1 at a time, still caused the same problems.
I updated my mobo and gfx card drivers to the latest.

I have run out of ideas, but i have installed rivatuner and downclocked my card to 400mhz from 600mhz...
Not an ideal solution but it lets me play games a little bit longer than otherwise....the closer i got to stock frequencies the worse it gets.... probably get about 20 mins when i get home from work and then it locks up and will do it everytime very quickly i reboot from then on.

Does anyone have any ideas? Close to giving up now as i cant afford to buy a whole pc, already spent over £200 trying to work it out...