I'm looking to build a computer with a cheap C2D chip now, but upgrade in the future when the prices of the quads come down. Will the soon to be released QX9650 be the fastest chip for a 775 mobo, or will a faster chip come out before Intel changes sockets again?

not for long!

its just a faster qx6850 which is a faster qx6800 which is an overlocked qx6700

buy the qx6700 now or q6600 and wait for the new chips with massive memory bandwidth increases!

hard to say, intel could easily make 3.4EE with the existing quad and 775, the qx7000

Anyone know if the Nehalem will have a 775 version? That will be cool!
It won't have the integrated memory controller, but still cool.

Yes, they will. 775 is wider used than 478, and yet 478 got prescott processors. People will have the chance to gradually move from 775, else, if they will need to upgrade board, cpu, ram and maybe even graphics at the same time, they might go on to AMD. AMD lost because of this. Intel knew beforehand this could happen, but even if they did not, they learned. You can't build core architecture without being able to learn (just in case Intel is run by some kind of AI).