I tried to post this yesterday but somehow it missed.

I'm going to Cambodia in a couple of months. I am confused about the currency used. What is its value compared to US$ Thai Bt or UK┬г. I cannot find this information anywhere. I tried Aol money conversion but it doesn't cover Cambodia. I would like to be able to work out how much I'm being charged. If I don't know the value it is too easy to be overcharged.
I believe it is sometimes required to pay (or is possible) in US$. What is the best denomination to take, 10. 20 or 50?
Many thanks.