the Sempron is basically the same as the Athlon 64, but with less level 2 cache.
however, video encoding is one task which will see a fairly significant benefit from having more L2 cache.
that, and socket 939 is advantageous over socket 754. it allows dual channel memory, and up to 4 DIMM slots.
however, in terms of upgradability, socket 754 will actually be around for longer. It will be used for future budget processors (more Semprons) and socket 939 will be replaced by socket AM2 (which supports DDR2, and in future DDR3)