That is bad advice from asnorton44...
You need to stick to photo and text use on your internet browsing. YouTube videos even at bad resolution take up a lot of bandwidth. With a 5GB cap, you can't use the internet for leisure.
My brother used 10GBs of data with his T-Mobile phone using unlimited data for 1 week. He would go to work (over night security) and stream Hulu and Youtube for about 5 hours a shift for a week (5days). That is about a 2.5GB average per hour of YouTube or Hulu. So with a 5GB/month cap, it doesn't leave you much room to do anything. I would setup all automatic updates to alert you when they are updates but not DL them. Also I would look into a bandwidth cap on your router so you don't go over and they start charging you $.10 a MB. I couldn't be on a 5GB cap...
64876357255 bytes = 60.42~ GBs in 4 days.
EDIT:
Since I posted this (7/28/10 @ 3pm) and this edit (7/28/2010 @ 7pm), I have used 1 GB of data...