How much bandwidth does online gaming, youtube etc take up?

So were on a plan with uncapped speeds and 20GB allowance per month (I live in NZ,no unlimited plans which is completely dumb.Well actually there is but they say they reduce the speeds inside the hours of 9am and 2am.like wtf?) anyway.
We keep going over our allowance which is super annoying because they put us on dial up(which shouldnt even be an acceptable standard anymore)for the rest of the month which I cant stand.
So im trying to figure out where our bandwidth is going.Does anyone know how much online gaming(ps3) would take up per hour? or youtube videos.
My sister blames me for online gaming,where as she watches documentaries online, and I know she at least used to watch movies online aswell.

Online gaming is next to nothing really YouTube will take up a lot though... I guess you can go into playback options on youtube and change it so it always plays in regular quality as oppose to HD/HQ Though I still find it hard to imagine you're using 20GB a month purely on YouTube. Brainkiller05

what ISPs are you guys using? I get none of that crap and could do what I please. But then again, I dont dl huge files, just music. Tjeremiah1988