Get used to eating noodles for the duration of your stay. We had some of the worst food in Cambodia at in the market stalls of Psar Leu and Psar Krom. Fish that tasted like mould. Rice that tasted like mould. Something that we couldn't identify that looked like mould and, yes, also tasted like mould. But the noodle soups were consistently decent and palatable.
The restaurant scene is pretty limited. Mekong Restaurant along the road to Battambang serves up a small menu of Western and Khmer dishes that are very-much mould free -- the steak and fries here were palatable. Mittapheap Restaurant southeast of the central square, just north of the hospital, serves Khmer food only -- it's really just a cafeteria for travellers passing through, so it's large but usually empty.
You might also try Mean Chey, one block from Psar Leu, for meals of spring rolls and pressed meats. There's actually a good morning coffee joint, that also has good noodles, in the Psar Leu -- no name, but continue north through the market and look along the right until you see the one place that's busy -- full of people all sitting in the same direction looking at the television.
The best option for noodles, at least in terms of atmosphere, is the riverside promenade across from the Rithisen Hotel -- also not a bad spot for a beer in the evenings.
Another note on the beer front: we found that 'extra stout' was somewhat popular here -- one made by Angkor Beer, and the another called 'Black Panther.' If you're a stout fan, try asking around for it. It tastes almost a little bit like Guiness. Just close your eyes and think of Ireland.