Troubleshooting a crash (Dawn of War II)

[Fusilero]

• AMD Phenom II X4 Quad Core 965 Black Edition 3.40GHz (Socket AM3) - Retail
  
• OcUK ATI Radeon HD 5770 1024MB GDDR5 PCI-Express Graphics Card + Colin McRae Dirt 2 Full Game + Ruby Doll
  
• Asus M4A79XTD Evo (Socket AM3) PCI-Express DDR3 Motherboard
  
• Geil Ultra Series 4GB (2x2GB) PC3-17000 2133MHz Dual Channel Kit
  
• OCZ ModXStream Pro 600w Silent SLI Ready Modular Power Supply
  
• Western Digital Caviar Black 640GB SATA-II 32MB Cache - OEM (WD6401AALS)
  
• Coolermaster Elite 335 Case - Black
  
• Sony Optiarc AD-5240S 24x DVD±RW SATA ReWriter (Black) - OEM

I just finished building the above PC,

I'm currently having an issue in Dawn of War II, where after five or so minutes of gameplay the computer emits a large shriek and then the screen goes black. After this the PC sounds perfectly fine but the screen remains black so I have to turn off and on again. This is the first time I've ever had problems with a self built PC so I'm wondering what steps should I take to troubleshoot the hardware (if it is a hardware problem, Dragon Age seem to work fine, this same copy of Dawn of War 2 works on my other self-built PC).

 

[BigBruiserAl]

Download Msi afterburner and keep on eye on the temps, i have read some 5770 crashing at 85c ....just something to rule out anyway