I'm still in the throws of making a decision regarding what board and processor to use...specifically, whether to go with a socket 754, or the 939. The first thing going through your mind when reading this is probably "what are you giong to use it for"? I'm not into games, but I will most likely wind up doing some video editing, and I will have a couple of CAD and electrical estimating programs, the rest will be fairly normal.
I know just enough about the two to be dangerous, and to realise that one of the key differences is the type of memory, I should at least tell you that I will be going with a Athlon processor. Which socket would you use, and why?
Remember...someday I could growup to be just like you!

939. lot better in my opinion and much much newer.

Both Socket 754 and Socket 939 motherboards use DDR RAM. That's not a difference at all.

Socket 939 motherboards use more advanced chipsets than Socket 754, and would suit the tasks you wish to perform better, if you plan to build an Athlon64 system.

i've got a sempron processor which uses the 939 board.

Mmmm? A 939 Sempron? I have to wonder where you got one Janine - they're not available on the market yet, but there do seem to be a couple of "leaks".