If you want to leave yourself in the best position for further upgrading definitely go for 939. On a 939 board you will be able to upgrade your memory to dual channel, your CPU to most future AMD CPUs including Athlon FXs, and your motherboard to an nForce4 board (even allowing for possible SLi upgrade
).
Until recently it was true that all the more expensive CPUs were on 939 and all the lower end CPUs were on 754, making 939 the expensive option. Recently however AMD released some new 90nm Winchester-cored CPUs (the 3000+, 3200+ and 3500+) on socket 939. These CPUs make 939 a much cheaper option. (
also they overclock really well ).
If you get a 754 board you will be a bit confused
when you come to your next upgrade.
Conclusion: you can high end CPUs on 939 only, you can get lower end 64-bit CPUs on both but the 939 versions perform better and leave you with better prospects for further upgrading.
P.S.
nForce4 will be great
but the asus a8v and abit av8 are good current 939 boards.