Click here for part2: from Kampot to Phnom Penh
For part3: from Phnom Penh to Lao border, through Kampong Cham, Kreatie and Stung Treng, here.
At the bottom of the article is the map of the whole Cambodian tour, click on the track to get the elevation profile.
For general hints and tricks to keep your budget extremely low when traveling Cambodia, here's another article.
Thailand Cambodia border (Koh Khong) to Batum Sokor - 108km
An easy 15km after the border you will cross the long bridge that will lead you to the nice town of Koh Khong (not to be confused with the island). It's a pleasant riverside city with plenty of guest houses and restaurants, including some western style ones. The atmosphere is more lively than in Thailand, the places are open until later (not very late) and Cambodians seem to enjoy their life more. There's a beach nearby and a mangrove forest that make for nice but not unmissable day trips.