i have this on PC & xbox. The game is probably one of the best I have ever played - they really managed to get the whole team working thing down to a T.
Regarding high spec PC - I have this just fine running on Windows using Bootcamp for Mac. My system is one of the unibody macs (now rebranded as the 13" pro), but even so there no means designed for games - but the game runs fine.
Peasant. Q6600 @ 4.6ghz Vista64, 8gig ram 2x XFX 285 gtx 1 gig. 1x 160 winders drive + 3x 750gig WD deathstar in stripe plus parity all water cooled. Not tried L4D yet must get round to it but i just cant tear myself away from TF2