If staying with Intel, by going by their past record, it's best to upgrade the whole lot at once. With their CPUs requiring new MBs and RAM, it's not wise to plan on upgrade paths when using Intel CPUs.
I used to get complete new systems often, from one of the first Pentiums, Pentium IIs using slot 2, to PIII's requiring MBs using the expensive Rambus RAM. Not any real upgrade paths with them. Then AMD came along and gained the lead shortly, and switched to that. And was able to just upgrade their CPUs when needed. And while using AMD, I've learned it's silly to keep buying the top.
My $500 system from over a year ago, buying AMD's 940 BE Quad when released, is still a great system today. Scoring 9.4+ on all Win7 Index, except for my Caviar Black Drives (5.9). I don't feel behind at all with the latest I7's being released today nor I doubt from a year from now either, and I don't even overclock it. Though I did OC it just to see how far it will go on a stock system, but realized it's not needed with such little actual gain.
With something that you'll upgrade every few years, should look at the best bang for the buck. And with most systems, upgrade the whole package, MB, RAM, CPU, and OS to get the most out of it.