CPU should be fine, it's the GPU to worry about
Two points: It's a Source-engine game. If you can play Portal, or Half-Life 2, or Team Fortress 2 on your Mac, then L4D2 should work fine too.
Second point is that Steam seems to be struggling a bit this week with lots of updates. I bought L4D2 on Tuesday night (GMT+1) but I got continuous "Server Too Busy" messages downloading until Wednesday morning. Game downloaded OK over the course of yesterday and I could play it OK last night.
(I've got an i7 late-2009 27" iMac with the 512MB ATI Radeon HD 4850. Source-engine games at native resolution and high settings (but with AA turned off) get ~50fps steady for me on OS X. Including L4D2)