I tried to play l4d the other night , that game is too demanding. I can't even play at all low at 800x600, i would get 1-3fps. I don't know what engine its using, but thats the most demanding game for my rig which i have come across.
in all fairness you shouldnt even be able to play it on that rig a pentium 3 and a 8400 thats slow
left for dead is optimised to the max, source engine is not demanding at all, i could boost through half life 2 on a 9600pro and a oc duron