And also, Source games work VERY well with old computers...so if your computer sucks, you can probably still play Left 4 Dead, and it won't look like total crap.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )