Well, if you want to build a "future proof" system today then I would look at AMD rather than Intel. Intel will be releasing their Nehalem CPU at the end of this year (the Extreme version) with mainstream versions coming out in Q1 2009. Socket LGA775 will die a sudden death when Nehalem is released 'cause it will introduce new sockets.
The bad thing about going with AMD now is their relative poor performance compared to Intel's CPU. That's why they are cheap. In addition, AMD is stuck in a rut, Phenom took too long to get off the ground and it hit a few stumbling block when it was released.
AMD's next CPU will probably be socket AM2+, but given their track record since the release of Intel C2D CPU, are you will to purchase a lower performing CPU now in hopes that AMD will hit a home run next time around? Considering AMD's shaky financial standing, I don't think they will have the money to invest into a "Nehalem killer" anytime soon.
I recently built a new PC and I decided to go with Intel's C2Q, the Q9450, despite Nehalem's mainstream introduction about 10 months from now.