1. You'll find about the same upgradeability with any S754 motherboard and most S939s. They'll take whatever their respective socket can accomodate.
2. See earlier: "There's not a whole lot of performance difference between A64 motherboards"....
3. Nothing has Soundstorm now, so you'll be looking at the same onboard Realtek codec for anything. Your typical onboard sound, which is fine for all but the highest-end speakers.
4. What kind of "efficiency" are you looking for? That's some new "key" feature of a motherboard I've never heard of.
5. You will never find a motherboard to fit "any" graphics card without taking a performance hit--one exists for Intel LGA775 with both PCIe x16 and AGP, but the AGP slot is run off the PCI bus. Decide what you want and then buy the board--if you need to buy a S939 CPU with an AGP NF3 mobo now but want PCIe later, then swapping the motherboard is your best option.