A hackintosh doesn't Just Work, which probably half the benefit of running OSX. Assuming you'll have the desktop for three years, and reading 200 post long hackintosh forum threads isn't your idea of a fun, suck it up and buy the mac pro. You only have to spend the money once, you've got to live with the machine for quite a while.

A netbook hackintosh is just about perfect - it's by definition not your primary machine, so when it's down because you installed 10.5.7 by accident, it's not a big problem. (or, perhaps I'm just trying to justify buying myself a netbook)