It depends on your situation. Are you going to be using DDR2 ram or DDR3? Also bear in mind that socket 775 is nearing its end of life, whereas the newer I7 architecture is just getting started, it will have longer use.
So on 1 side, you have lower costs with socket 775, and the ability to use some older ram if needed, on the other hand higher costs and no risk of being obsolete in a year.
Myself, I went with the AMD Phenom II 965 Black Edition, using OCZ DDR2 1066 memory and can build on this as the Phenom II AM2+ and AM3 platforms are also very young like the I7 platform.
I still run 2 socket 775 systems with DDR2-800 ram on Windows 7, I expect them to be running for another 3 years yet, but I am doing no further upgrading on them. They have everything they will ever need...
Hope this is of help, and good luck on you new build...