The main problem is the hardware support in Leopard. Cause macs are sold as complete systems, Mac OS doesn't (need to) have drivers for other things like motherboards, graphics cards, etc. That's why this solution is good cause the graphics processor is the same as the macbook, and therefore works, and it's tested. If you want to add another graphics card, make sure it's one you can find in a Mac Pro or something so you know it works with Mac :)

Processor and ram should still work as long as it's from the same series, but will likely work anyway. Especially ram. Same goes for harddrive, dvd drives etc, there's no restrictions there.

I might build this myself if I can sell my stuff that I'm selling... I'll use it as a storage tank probably cause my macbook really runs out fast :P I'll post if I do, but I think someone else might have money faster (beaner?)