My shitty PC can run HL2 and various source games absolutely fine. O http://www.youtube.com/watch?v=AnK3VguvCuQ
Zombie Master, a source mod, in a graphically intensive map, with 20 zombies in front of me, and FRAPS recording a video of it all. Only drops into an uncomfortable FPS range once. For the rest of it its perfectly playable.
I know, I keep forgetting how low the system specs are. My old 9600XT played it nearly maxed out at 800x600.