Since I’m in the mood to document various hardware modifications, here is a link for the archives:
http://www.katastrophos.net/macosx/9000pro-mod.html
This is a tutorial written in German for a mod I did way back. It describes how to modify the case of an Apple Cube to actually insert a better graphics card along with proper cooling of the components.
Much of the article deals with how to relocate the DC/DC board to the inside of the Cube to make way for the bigger graphics card. It goes on to describe how to modify the graphics card with a better heatsink. Finally, it shows how to cut the outer casing for proper cooling.

You might ask, why this is called Biohazard mod. Check out this and that.