I can smoothly run it on my P4 HT 3.0 GHz / 2GB RAM / X1600 Pro 256 MB
I pressed the recommended settings button, then turned up the resolution to 1280x1024; anisotropic = trilinear; v-sync off;
no problems with great amounts of zombies --> perfect
Pentium D Dual Core 2.8Ghz
RAM -> 2GB
ATI Radeon 1950GT Sapphire
19" Digital Monitor @ Max Res for the Graphics card
Runs really well, better than TF2 for me actually. The only time I get the slightest lag is when the horde are all running down a hallway at me while me and my team mates are shooting. And its only about 20FPS still.
It runs much better than I expected. Been playing it on my old comp at low/medium settings and 1024x768 and haven't really noticed any slowdown at all. The specs of that comp are:
Athlon xp-m 2400+ @ 2.2Ghz
GeForce 6800GT
1GB ddr2
Windows Xp
In the beginning it seemed to stutter at parts, probably due to it loading all the map stuff for the first time, but after the first playthrough the stuttering vanished altogether. It looks quite ok for being played on low/medium settings and a low resolution too. I'd say you don't have to worry about the game running even if you don't have a top of the line computer. At least the demo levels work great.