Hi,
The US dollar is still widely used and accepted and is the de-facto operating currency in Cambodia... Thai being said, the UK pound and Euro are also widely accepted and quoted by forexs' in town... Thai baht is also readily accepted at most border towns..
You will not find it hard to locate an ATM that dispenses in USD...
As far as exchange goes, there is no need to buy any Riel until you get to Cambodia... You will only use riel for very small purchases (such as a short moto rides) and you may receive riel back only when the change is less than USD$1 or the merchant does not have sufficient USD notes on hand...
Be sure that whatever foreign currency you choose to bring to Cambodia that you make sure the notes/bills are clean, not torn, damaged or marked.. Some exchange shops are rather picky when it comes to accepting larger denominated foreign notes and may refuse them if there are marked, torn, excessively soiled or such forth...
Sadly, you will often get back as change the dirtiest, worn out, torn and soiled riel notes you've ever seen...