It all depends on what you want...the socket 939 AMD 64's are the future. However, you can get a great deal on a socket 754 right now. I decided to go AGP/754 instead of PCI-e/939...
That was based on my tendancy to tinker and build every 18 months or so...I just dont think that the "upgradability" of the 939 is a sure thing. Since SLI under PCI-e is program dependent (as of now) how can you tell if it'll be worth it.
All in all, if your planning on a "3-5 year" box...go AMD/939. If you think your going to tinker sooner then I'd go 754. I'm very happy with the DFI lanparty board (754) I got. Matched up with a gigabyte X800pro I'm set till the itch returns:)