If you want to save time waiting and lining up on arrival you can also get your Visa e-mailed to you in advance at: http://www.mfaic.gov.kh/evisa/

It's USD$25 and you need to upload a JPEG portrait shot of yourself. Doesn't need to be a proper passport photo or anything. Just get any holiday snapshot and crop it so it only shows your head. They'll accept it.

I had my VISA e-mailed to me within 24hrs. Print two copies and slip them in your passport.