I traveled that way in 2001.

Here was the border post back then:

www.flickr.com/photos/gee...6323329569/

But things have changed tremendously in Cambodia since then. Growth of tourism has been incredible. It should be easy and fast these days. Do you have a guidebook? It should describe the route in good detail.

I went through that border in 2006-7.
But something to keep in mind, back water, land borders or seldom cheap or easily facilitated. Often, if you can, taken the most frequented air port; the same as all the middle class lazy tourist do, is the cheapest and smoothest.

I bought an expensive visa for Laos, made the trip up the Mekong, and found that our driver and the border officials wanted bribes. Still it was a shorter distance to that over touristed back packer haunt which I wont mention here.

I came over land to Cambodia being the adventurous sort, and realized with all expenses, the stress of fighting with border officials for bribes, that it would have been far cheaper to take that flight from Bangkok to Phnom Phen and then the bus down to Siahnoukville.

Still it was an adventure.

L