I see this is a fairly old thread.
I've built a lot of "ghetto computers". Not as imaginative as what I've seen here, but ghetto none-the-less.
The system pictured below is my back-up gaming rig. The Dell Dimension 2300 case was fished out of a dumpster. CPU fan is socket A modded to socket AM2.
It currently houses an MSI board purchased at low cost due to cosmetic damage, a Phenom 9750, a GTX295, and 4 GB of memory.
To cool it down I have the metal mesh panel that goes on one my Antec 900 bays and small fans mounted to that, blowing air into the case from the CD-ROM bay. I then have a small internal fan that blows air from the bay toward the video card intake, to keep it cool. The exhaust fan came out of an AT power supply that was probably 15 years old. The front USB ports actually work and were stripped from an e-machines case. The power buttons and LEDs all work just like they did originally.