After surfing the web. How can i know how much data has been downloaded/transferred from the net?

Ok this is pretty simple its just a matter of looking in your harddrive and your cookies, so were going to open My Computer and click on C:\ Drive and then Documents And Settings then w/e your user name is then Cookies and you can tell how much cookies you have but before clicking on that you said files and things such as that so just click on local settings it maybe hidden so make sure you make the folders unhidden and then right click on Temporary Internet Folder and click properties and it wil give you the amount of space taken

An average YouTube video (assuming it's just a music track) would be about 7 MB or roughly 300 videos for 2 gb - 75 videos a week, just over 10 a day basically You'd still need to leave something for general surfing, so 6 - 8 a day maximum and a lot less if they are high quality ones