On my quad core rig in my signature (with 4GB ram, not 2... I need to update that) I am getting around 20-30 FPS when things get busy. It still plays ok at 20 fps, but you can't tell you're at only 30fps when you are there. I don't know why some games are unplayable at 30 fps, and others are awesome.
Settings are all ultra (enthusiast) except shadows, which are on high, and Textures are on medium. This is at 1280x800.
It seems to run a bit better than the first one atleast.
New Rig: Intel Q6600 (2.7ghz), ASUS P5KC, 4GB DDR2-800, Palit 9600GT (1GB), Ultra X3 1KW (thanks LR!), Vista 64bit
Dell Rig: AMD Athlon X2 4000+ (2.1ghz), 2 GB DDR2 Ram, MSI 8800GT (512MB), Vista 32bit