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but ultimately decided I wanted more drive bays.

I'm still hoping for an AMD Shuttle in the P series chassis. 4 hard drive bays, media card reader and the optical bay. That, and PCI Express, all in a newer designed layout that deals with thermals better, and quieter. The only P Series boxes so far are Intel based.

Regarding the AMD sockets:
754: Pretty much dead at this point as others have said.
939: The goahead consumer socket. Possible upgrade path to dual core processors.
940: The server socket, for dual and quad systems. Already shown in public to support dual core processors by taking an off the shelf Proliant with a different firmware load and the prototype dual core chips in it.

The dual core aspect alone would have me eyeing a 939 system. While it is not 100% confirmed every system will be able to just swap a CPU to upgrade, it's definitly a 0% chance on socket 754.