After much time lurking on FT I am prompted to post, as this is the first time I have seen PNH mentioned here!
In answer to your question, yes, it is possible to go PNH-SGN by land (bus/taxi) or even by a combination of land and boat. I haven't done it, but know people who have.
The distance isn't far, but the state of the roads and occasionally long lines at the border mean that the trip can take a whole day. Most of the buses leave very early in the morning; I understand that they get into SGN rather late in the afternoon. I hear the boat trip (via the border post at Chau Doc) is more enjoyable.
A website with pretty good information on overland travel in SE Asia is:
http://www.talesofasia.com/cambodia-overland.htm
Happy travels,
Michael