whats a slimline? laptop? and i think you mean 220gb of storage, not memory
cod4 or any other game is mostly dependent on the video card, but cpu and ram are important too. nobody laugh but i play on a P4 2.8Ghz, 1gig ram, and geforce 7600. sometimes my framerate drops to 20 fps and it sucks. its a huge disadvantage but its still playable...sort of. personally that's the lowest system specs i would play on and only for casual play. i stick to smaller servers and never use smoke

hardest thing to do is hit someone while they're running.
I'm all for someone switching from console to PC but I'm pretty sure you can get a keyboard and mouse for the ps3 if that computer doesn't work out and you don't want to upgrade.