Wait a bit, don't rush out and upgrade your system just yet. Nothing short of a render farm is going to fix your framerate issues with this game as it was released before it was completed.

I have a Core2Quad at 3.2GHz, 4GB of RAM at 1066 and an Nvidia 8800 GTS 512 on Vista64 and this game runs like ass whereas everything else I own runs like melted butter over a smokin' hot 18 year old catholic schoolgirl's arse.

Wait for the game to be fixed before making any upgrade decisions if at all.