I would advise staying away from orphanages while in Cambodia. An entire industry has grown around foreigners who think they are helping but are creating a much worse problem. Children who have families are being sold or rented out to orphanages to supply the demand that has been created by this misguided tourism industry. On top of that, actual orphans are already coping with abandonment issues. Having someone come into their lives, show them love and affection, then leave to go back to their home country can cause even more psychological damage.
http://www.thinkchildsafe.org/thinkbeforevisiting/
I've been volunteering at a wonderful school in Phnom Penh for the last few months. It's a great organization that I can recommend highly, but you do have to give them a three month commitment. If you're interested, PM me and I'll give you some more details.