Computer Buying Cycle

I would like to buy about every 4 years. But now with the release of the intel machines and the easy ability to run windows applications on them I might rethink my strategy and buy something sooner, ie. this year even though my computer is only 2 years old. But once I do get the intel machine I would like to keep the cycle to 4 years or whenever the raw speed(Hz) is increased 4 fold (counting cores independently). Whichever comes first probably.