Siem Reap is Cambodia’s biggest tourist destination. Deciding how to use your time at the temples is a complicated business.
Some good resources for this are
Particularly useful is their suggested itineraries from 1/2 a day through to a week Canby Publications Also their links to maps and other information is unrivalled.
An old favourite is packed with information http://www.lonelyplanet.com/cambodia/siem-reap Their forum with travellers asking and answering questions can be an excellent source of current information ThornTree Forum SE Asia mainland
TravelFish is an Asian specialist site and is packed with information, not always found elsewhere. http://www.travelfish.org/country/cambodia Their form is another source of lots of questions and answers http://www.travelfish.org/board/topic/cambodia
A very good narrative of 72 hours in Siem Reap http://www.rustycompass.com/destinations/category/city_6-cambodia-siem-reap/18-our-ideas
Places to go, where to eat and suggestions for accommodation all possible with 2 days in Siem Reap 48 Hours Siem Reap – the telegraph
If you only have a short time to spare, this is a whistle-stop set of suggestions for a day in Siem Reap. 24 hours Siem Reap Sydney Morning Herald
Have a comprehensive list of suggested itineraries for a range of travel periods. http://www.angkor-guides.com/siemreap/suggested-itineraries