Realistically there is no set answer. The way technology moves will determine whether upgrade or a new machine makes more sense. For example, my current machine has AGP 8X graphics slot. Now, PCIe is standard. It makes no sense to upgrade my machine now at three years old. It would make more sense to get a new motherboard, power supply, processor, video card and one of the new hybrid hard drives. With that many parts it is essentially a new computer.