As far as I am concerned, this game is nearly unplayable without a quad core. My dual core AMD processor is almost always at 100% cpu usage on both cores with this game. FPS are pretty poor and there are delays, stutters, and sound problems. Perhaps people with more optimized or merely better dual core systems are running the game fine, but for me there will be a substantial investment in hardware in the future to be able to play this game in the manner that it was intended.
You can not exchange your mobo in your DellE521 because it is a BTX board!!!!
I have more a less the same rig as you, you can overclock your DellE521 by using clockgen and stay away from Vista!!!!!!
CPU: AMD Athlon 64 X2 6400+ OC to 3.5Ghz
GPU: Sapphire HD4890-1Gb VaporX
RAM: 4gb Crucial ballistix PC2-6400
PSU: Corsair TX750W
WOS: XPsp3