As the other posters have said, you need to decide whether driving yourself in Cambodia is for you, given the challenges and risks.
However, if you have previous experience driving in the region then I'd say go for it and in fact, I would actually recommend renting a car rather than a moped/motorcycle as if you fall and have an accident on one of those things, you'll be sorry. Whereas by driving a car you'll be safe unless you're involved in a very serious accident.
AVIS Cambodia (formerly known as AVR) rents out a multitude of different vehicles with and without drivers. If you're not sure about your driving skills in Cambodia, why not rent out a vehicle with driver for a day or two to see if you're confident enough to get the hang of things, and if yes, then you can self-drive from then on? A driver will add another US$25-35 or so to the daily cost.
For more information, go to their website: www.avis.com.kh.