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Socket 754 because it is cheaper and provides excellent performance. Everything else will be extremely expensive and not very secured (DD2, PCI-e are still at beginning stages). The major downside is no future-proofing.
The highest cpu for the 754 is 3700+ but you can just oc it. 939 supports dual channel mem but 3-5 percent increase in performance does not justify its price.
we need much much more info... i mean a pretty desecent entire system based on a64 would be out of your range, and dont even go to 939 and 775, i mean if its just the mobo and the cpu, yeah, but if you need a system... you gotta think about monitors, vid cards, memory, psu, case, ect...