Running Mac OS X On Standard PCs
Indeed. You're not legal if you buy a copy of Leopard. It shouldn't let you sleep any better, as you're still breaking the law.
Yada yada mid size tower, yada yada I want to upgrade. Yada yada I can build XYZ for ABC less than what Apple charges.
Tough. At the end of the day to be legit you've gotta pony up the cash for the MacPro, accept the non-upgradability of the iMac or Mac Mini, or not run the software. /me waits for the obligatory "or buy a Mac laptop!" reply, so to reply to you before you reply: you missed the point. Entirely.