Welcome aboard!
I'm running SH3+GWX3 plus a ton of mods on a newish desktop, about a year old, but with an 2.81 GHz processor, 4 G of RAM, and ATI Radeon HD4650 1 G PCIe gfx card. Windows XP SP3 O/S.
I don't know how much difference the processor makes, but the other specs you mentioned seemed pretty close to mine. And this rig handles the game plus everything I've added just fine.