What you are using the machine for would help.
Trying to future proof over this kind of time frame is impossible. A Core 2 Extreme ($1000+ processor from 2008) gets slaughtered by the $250 mid-range i5-2500k from 2011. So in three years, cutting edge processors will be outclassed by mid-range hardware.http://www.anandtech.com/bench/Product/48?vs=288
This is the most extreme example, a 3770k (which is on the line between mid and high end in the whole scheme of things) wont last near as long.
Buy a processor that will give you the performance you need now, not the performance you hope it will give in X many years. It will work out cheaper (and likely more end up a more powerful rig) to upgrade the processor when you need to.