Rob Griffiths of Macworld.com built a sub-$1000 Mac Pro clone. (A Mac Pro is on the left, Rob's Frankenmac is on the right).
After all of the parts arrived at my home, it took a few hours to build the machine. If you've never built your own computer before, it's an interesting experience–there's something quite satisfying about putting it all together, powering it up, and hearing that first "beep" that lets you know you haven't just turned your collection of parts into a collection of junk. Of course, if you don't hear the beep, there's an entirely different reaction, one that borders on panic. Thankfully, I heard the beep. But building the hardware is actually the easy part of the process.
Next, I installed Vista on the PC, just to be sure everything worked. From there, it then took many more hours to get OS X working right–while the process is relatively straightforward, there are a lot of steps involved, and BIOS settings to tweak. If you want to run Windows and OS X on the same drive, there are more hoops to jump through to get it all working. But after many hours of reading, assembling, disassembling, screaming, installing, uninstalling, reinstalling, saying bad words, pestering friends, and generally not having very much fun, I was done: my machine was up and running, and capable of booting into either Windows Vista or Mac OS X 10.5.2.