I just went through the same decision process yesterday. The socket 754 is EOL'd for everything except the low end consumer chips. Socket 939 is supposed to be where it is at for at least the first half of '05 for Athlons. There won't be any new Athlon 754s so if you go that route I'd buy at the higher end of the spectrum today rather than plan for a minor upgrade later on.

Personally I went for the low end socket 939 Athlon 64 (this is for my personal file/web/video/etc server running FreeBSD). It is already a huge speed improvement over the 5 year old celeron system it is replacing and if I decide I need another upgrade in a year or two I know that I'll be able to get a new CPU that is at least 50% faster than this one so the upgrade may actually be worthwhile. The price difference was only like $50 compared to an equivalent 754 system and worth it for a little future proofing in my opinion. Not to mention your getting a newer motherboard chipset in the process with additional features.

-Mike