I really don't see the point of this, beside wasting a considerable amount of time and being able to boast about it. If you build such a machine you'd better go with Linux or Windows; the real advantage of OS X is in the tight integration between hardware and software.
Apple's Mac Pro, the sleek and shiny trash can from outer space, is certainly a feat of engineering. It also costs $3,000. If you want to build a comparable machine yourself, you can save a lot of money by going with a Hackintosh.