I received CoD4 for Christmas and played through the single player in about a week or so of nightly gaming.
One word: awesome.
The variety of missions (assault, sniper, AC-130 gunner (Hells Yeah!), etc), the graphics, the sounds, all are first class. The online multiplayer is also a hell of a lot of fun. You can customize your online "class" when you play to pick the gun of your choice, with the addons of your choice (silencer, 'nade launcher, red-dot sight), with the attributes that suit you (run faster, steadier shots, more weapon damage, etc).
I did just build a new rig just to play the latest 5-star games: quad core 2.4Ghz overclocked to 3.2Ghz on air cooling, Nvidia 8800GT 512MB card, 4GB ram and can play most everything but Crysis on 1600x1200 with AA. This was quite an upgrade from my 3 year old Athlon 64 2.2Ghz, 2GB ram, 6800GT. I did play the CoD4 demo on my old rig at 1024x768 res and it played fine on that.