550w..it only happens in this one particular game

I have the full game..I used the demo in the both pictures but it still happens

I have 8GB of RAM

WHATS WRONG ITS SO LAGGY EVEN ON HIGH SETTINGS...

System Information:
Intel® Core™2 Quad processor Q8300 2.5Ghz
XFX - ATI Radeon HD 4890 1GB DDR5 PCI Express 2.0 Graphics Card
Microsoft Windows Vista Home Premium 64-bit Edition

Some pics

http://img716.imageshack.us/img716/2159/arma2demo20100125015622.png

http://img684.imageshack.us/img684/3131/arma2demo20100125015624.png