I did it by myself, got back from Don Det island, took a local bus to the border, got my visa there, and started walking towards Stung Treng (as it was in the afternoon and taxi's try to rip you off). after 5km walking a local guy turned up and took us to Stung Treng for less then nothing. Though you do don't want to do it this way as you might end up without a ride.
book a bus in Pakse or 4000 islands to Stung Treng, trip is really short, from Pakse to Stung Treng took me about 4 hours, Don Det to Stung treng, about 1,5 hour, not that far or I might have been lucky to be on local busses that drive to kill!
ah well, visa at the border: no problem at all!
Maybe wasted advice (if you've already done it since posting on 20/12/2010) but you're probably crossing from Pakse or the 4000 Islands into Cambodia. Pretty much every agent or guesthouse/hotel there can book your ride, but don't book a ride only to the border (where you are at the mercy of many opportunistic vultures). Book transport (van, pickup, whatever) at least as far as Stung Treng or Kratie (but not to Phnom Penh which is too far same day). Transport prices should be around $15-20 (USD) to those recommended destinations. Then pause for a night and decide where is next since you'll then be on the proper road network with a choice of buses. Going further check out transport options on the link below, and better to buy bus tickets at bus stations not agents if you want to save a few bucks.
Bus info here:
Big map here:
thanks again for all of the above advice especially @Captain_Bob for the map and bus info, we still haven't done this part of our journey yet so all information is always welcome.
How much would I be looking at for the visa? and transport to Stung Treng all in do you think? this would be for 2 people