Take a look at http://rectravel.com/rep/

Also, http://groups.google.com within rec.travel.asia . The most authoritative resource is George Moore.

The Angkor site is world class and well worth visiting. I would go from BKK and not Vietnam, however, since you mention it's your first trip to South East Asia.

Plan on 3 full days in Siem Riep, the "launch site" for visiting the Angkor complex. Add a day each way to travel in and to travel out. Possibly add a contingency day, which can be used for a boat ride on the Tonle Sap lake.

Consider getting the Dawn Rooney book on Angkor, http://www.amazon.com/exec/obidos/AS...419996-0113723
. Too bad the latest edition doesn't use Michael Freeman photography, which you can see in Angkor: Cities and Temples http://www.amazon.com/exec/obidos/AS...419996-0113723

You are at some risk of drug resistant malaria traveling overland. No worries when traveling by air into Siem Riep and visiting the Angkor site. Take a look at http://www.cdc.gov/travel/seasia.htm

For a hotel in Siem Riep, my wife recommends the Angkor Village http://www.angkorvillage.com/indexhr.html (I usually go more low scale, such as the Golden Apsara.)

Get a guide and travel by moto. (This is somewhat harder to do these days, since guides usually want to travel in a car.)
Rich