One of the reasons to buy a mac pro is upgradability, and that's very important to me. I know that the CPU can be upgraded, but to what extent? For example, if I buy the base quad core and in 2-3 years I want to upgrade to a dual-cpu system, would I be able to or would I have to replace my system? Sorry if this question seems kinda trivial, haha. CPUs aren't my strong point as far as hardware goes.