I'll second this. I built a hackintosh 2 years ago that ran 10.5 beautifully (leopard), with quad core, 4GB RAM, 750GB hard drive, for only $800. The problem is that now there is no driver support to run snow leopard, so I'm stuck running an OS from 2 years ago that probably won't be supported much longer, and I can't even install point releases very easily without risk of breaking my system.

So, 2 years later I've got a system that runs Windows or Linux beautifully but isn't really a functional Mac. This can be great for a short time, but just keep in mind that anything you build now will probably not run 10.7 when it comes out next year.