To me, the right thing is either LGA775 or AM2 sockets.
LGA775 is for Core 2 Duo mainly, and new CPUs will be released for it for a while yet.
AM2 is for AMD CPUs from the Athlon X2 upwards.
It will also have new CPUs released for a while yet, and will also be compatible with the next socket, AM3.
So go for either one of those for maximum upgradeability, and two PCI-EX16 slots.
The 939 motherboard above has the second-best CPU for it already installed.
See what I'm saying?