Go to speedtest.net and check your UPLOAD. Not sure what it's supposed to be per player for CoD4, but for BF2 it's about 80kb/s (which should give you some leeway, seeing as that game has more objects in game to track).
So if you want a 10 player server you'll need 800kb(it)/s, constant while the game's in progress. You can get some nasty lag spikes on home connections though. Give it a bit of extra headroom, incase everyone else on your contention block decides to hit the same youtube vid at once.