754 pin is for "older" Athlon 64's with single channel memory controller.
939 is the newer socket for Athlon 64 with dual channel memory controller, and more future support (ie: Athlon X" - dual core chips).
Socket 754 is good for a budget system, but you will be limited in the future, as AMD will probably not release future processors for it, and definately not dual core processors.
939 is the best bet for a new system