Angkor Wat and the other ruins might be one of the highlights of your trip. They are probably the most amazing thing I've seen in my life. You can easily spend a few days checking them out, even if you get templed out. I was there in March 2002, and it was dry and hot. I've also heard that it's gotten more popular as a destination, so you should plan to get up early if you want to beat the crowds (and the heat).
Anyway, you can get lots of practical advice on Cambodia from www.talesofasia.com.