One of the things that will either make or break your trip is food, and whether it's fit for your conmsumption. Food poisoning can be a nightmare, and can ruin your whole holiday. In Siem Reap, apart from Khmer food, there's also western food. You certainly won't starve in Siem Reap.
If you keep counting your riel (Cambodian money) every night, then you should eat at the market at rock bottom prices. The market is very close to "Shadow of Angkor Guesthouse" in Siem Reap. But make sure you've a strong stomach. Speak English here, and you'll be the star attraction.
This is a Chinese restaurant where I had my dinner a few times. Friendly staff. As bees to honey, Chinese people get excited whenever they see the word "Beijing".
Feeling rich, and want to splurge a bit? Cafe Indo Chine will lay the red carpet for you, especially if wads of US $ are sticking out from every part of you. This is one of the nicest cafes in Siem Reap.