This is my current rig:
Abit KN9SLI MOBO
AMD 64X2 Dual Core 5200+
4 GB Dual Channel DDRII RAM@800Mhz
Nvidia Geforce 8800GTS
Audigy 2 Soundblaster
And I still get crappy framerates if there's a big explosion, with lots of fire and smoke
But, like was said before, if your rig can run stock SHIII, it should be able to run GWX.