I agree with net4gains, 1GB transfer is a lot if you are just writing emails and searching for stuff on google. But once you start getting into media, it can add up quickly. And with gaming you will be up there.
If i were to guess, from this question's page.. there is like, what? 8-9 pictures, and they are pretty small, so say 4-5 KB each, if that... then you have all the code that is passed through it.. so maybe like 20-30 KB.. lucky for caching if you refresh the page it wont reload all the images. So if that is all you do then 1 GB is more than enough.. but if you think of various games then you are talking like 500-600 KB every few minutes, sometimes more depending.
hope that helps