Intel released the socket 1366 earlier this year, and a month ago the socket 1156. The new i5 and i7 CPUs are what Intel will focus on, and there will be no new LGA775 socket chips from now on. This means the prices will rise as these become legacy items, and any further upgrade will require a new motherboard, RAM, and CPU.
By choosing the AM3 or i5/i7 route now, you will have the ability to just purchase a new CPU should you feel the need, instead of replacing all the guts.
Not to mention both the i5/i7 and Phenom II 720 out perform comparable LGA775 chips.