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The key is to spend a little more for future capabilities. For example, try to look for a motherboard with 800MHz FSB (if you're interested in Intel processors), that way you can save some cash on the cpu now by getting a 533MHz FSb and deferring the cost of the upgrade for later, when the 800 FSB chips are cheaper.
My philosophy is to be on the bleeding edge with motherboards and on the trailing edge with processors and RAM, that way you have some flex room and can have some serious speed boost in the future without spending too much. Make sense?