It runs perfectly fine on my machine, which is lesser than you first two's. (sorry bad grammar)
I have a 2.26ghz pen4 533fsb, 768mb RDRAM, and a GeForce4 Ti4600 and I can run High 1024x768 decenlty enough (20-40 fps). I can easily get 60constant fps if I just lower the resolution. For the biggest performance boost, turn off AA and AF, and turn off shadows in the advance menu. This should help you a lot.