well windows os will take up 20 gigs, depending on a game its between 5 and 20 gigs
the biggest games i know are: battlefield 3 that is around 20 gigs, la noire that is 20 gigs too, max payne 3 will be 30 gigs.. i would go for 320gb
The 120 I didn't think would be enough space. The 160 was 4 dollars cheaper than the 320. So I decided to get the 320.