I use an internet Dongle at work, so i'm online probably about 40hours a week with mine, i have 15g allowance and don't go near it, maybe around 5gig if i'm on a lot
Well i got 10gigz aswell. Haven't been playing for a full time, but a few hours won't use much. 20gb will be enough aslong as you dont plan on downloading large files and wow updates. Guildwars is ~3 MB p/hour, wow won't use much more.