in your situation it sounds like none of the options are really worth it. cod has about a 6 hour campaign. depending on difficulty. not worth it if your only gonna play it as a single player game. halo is like some one else said without the custom maps and things it lacks a lot. but i hear good things about the campaign(better than cod) SC2 is amazing but if your having issues with low frame rate its probably better to push off till you have a better computer.
so that leaves you to games dedicated to single player. as they are probably what you would like the most. since you don't have gold. my suggestion is Mass Effect 2. Dragon age 1 or 2 although 2 is a much faster paced game but is very repetitive in the areas you go to which gets annoying. you could get fable 2 or 3. there are tons of games and i would suggest them over those three any day given the quality of your pc and that you don't have live.
Joey-Kazaam
get a new pc, then sc2
what league r u in anyways? if you're in bronze or silver i wouldn't bother though :P lol i jk