get an am3 cpu on am2+ board, the am3 boards which are out now aren't worth it due to future incompatibilities so there's not much pointing in spending the extra cash.
775 won't have any upgrade's and i7 isn't a very good bang for buck and will cost more than the other systems.
i was thinkin am3... what future incompatibilities are you talking about though? i know there's some new chipsets that are gonna be released, but old chipsets will still be compatible with whatever new am3 cpus they release, right?
for am3, i just thought that it'd be better futureproof-wise, since you will only have to upgrade cpu (and possibly ram), but not the mobo?