If you are going to be gaming then you would like one that has fast access, WD generally does. I'd go for the 8mb cache for that.
If you are going to be downloading a lot of (linux distro's as an example), then you would need space. If you have a CD-burner then you could make do with 80 Gig, but 120 might be better for you. It all depends on how much you are willing to spend.