Mac, the Live Account for Games for Windows will cost you $49 US per year. That does allow you to keep a record of your rating compared with other gamers on Live. Now, you might check the games themselves. I know, for example, that Halo2 and ShadowRun have a card in each of them that's worth one free month. So, with those two games, I can activate an account and be good to go for two months for free.
BUT, I really don't play online games so much anymore. Everyone wants to use Voice Chat and my deafness makes that impossible for me to use. And, it only frustrates those who don't understand the situation.
EDIT: Now, let me add this about CoD4! This is, without any hesitation, one of the best coded games ever! I'm playing this on my DX10 card under Vista 32-bit @ 1280x1024, high settings, etc. It's totally AMAZING!!!
There's one battle at a TV Station this is total MAYHEM/Massacre!!! And, I just finished a mission where I got to use a Stinger to take down a helicopter. Missed it the first time...got it the second time.
