In the context of using mobile broadband plans with an upper limit (in this case 5GB), it's useful to have an idea about how much data transfer the different kind of internet media will require. Basically, as a rule of thumb, you can say that the more complex or higher the quality an internet media is, the more data will be transfered when you use it. Below I've tried to divide different types of typical internet media into three categories. The higher the data transfer requirements, the faster you'll be hitting your upper limit on your mobile broadband plan.

• Low data transfer requirements
  • Text chat.
  • Simple web pages (like http://www.google.com frontpage and Google result pages).
  • E-mail with no or small attachments.
• Medium data transfer requirements
  • Low and medium quality sound (mp3's and net radio)
  • Complex or content heavy web pages (for example http://www.nytimes.com/ or http://www.flickr.com/)
  • E-mail with large attachments. 
• High data transfer requirements
  • Video (for example http://www.youtube.com/ and web cam conversations).
  • High quality sound.

The categories given above are just examples. As you can see, an internet media can exist in more than one category, depending on the quality. I have no figures to back up my claims, but if you use media from the "Low" category, you'll almost never run into your 5GB limit. You can use media from the "Medium" category a lot (and I mean a lot!) before reaching the upper limit, but it's possible. You should be more cautious with media from the "High" category: I think I once saw that a 3 hour web cam conversation on Skype used about 0.6GB of combined data transfer of upload and download.  

I hope you find the information above useful! I you have any additions to this subject, you’re welcome to post a comment. Cheers!