You'll enjoy the sights in Cambodia. The temples of Thailand never inspire me however the Angkor period temples of Cambodia are always of interest.
Looking forward to seeing where you explore about enjoying the backwoods of Cambo. Depending how far off the beaten track you intend to get I would recommend other than the 650.
It can get tight, and if the bogs have not dried up before ya land real mucky. If however its mostly fireroads to the temples you're planning on accessing then most anything will do. Sadly the country is changing rapidly with the Chinese influence developing the infrastructure. What used to be fun dirt trails are regularly getting bulldozed, graded, and even asphalted. A real shame as the character of the land is changing quite rapidly.
Looking forward to reading about your adventures. If you need any local information there is a website that a lot of people are using for info in the area: http://www.rideasia.net/motorcycle-forum/forum.php
Best of luck on your journey