I stumbled over a pretty nice video log of a guy taking a regular PC computer, building it into a Microsoft Xbox case. Then he uses the EFI-X hack to turn his PC into a so called 'Hackintosh', which is a PC running OS X.
The creator Will Urbina, states that the total cost of the project was about $1500, which is much cheaper than buying a Mac with the same performance. To to build it into the Xbox case, you need some other skills.
