We are worried! P.S. We have netflix on both wiis and we are always watching a movie(which needs the internet) So yeah. hahaha
You are right to be worried. If you watch HD movie streamed from NetFlix data usage might be as high as 1GB per hour. NetFlix is sure to send you data usage up over 5GB and perhaps quite a lot higher.
Most ISP's have a way to check your internet data usage and you can usually request the ISP send you an email when you're approaching your plan limit.