I'm playing L4D on a P4 @ 3ghz + 1GB ram, pretty good enough on low settings.
However my setup fails on very intensive scenes because the 250watt psu wasn't meant to be powering the 3850 AGP card it currently is. Doesn't happen often but if the card demands too much power then the computer will crash and needs to be drained of any remaining charge in the capacitors before it will restart (unplug power cable, press the power button on the computer, reconnect power cable).
Just one of the many reasons I want to make a rig from scratch now :]