if you are having trouble with your graphics.. even thou you have a good rig like that.. you might want to check if your Graphic Card Drivers and DirectX version are up to date..

I'm using a winVista + 4GB RAM + nVidia 9600 (a budget graphic card), I still can do 1024x768 at Medium graphic setting and play just fine

and IMHO, I dont think resolution plays too much difference in the game viewing size because I've tried going to higher resolution and the game space is still the same