I'm not real sure why you'd want a cube. I just bought a 1.2Ghz iBook, an external USB 2.0 drive enclosure to put my PCs 120GB HDD into, and ordered an iBook bookend docking station. I'm trying to consolidate having 2 PCs and one PC laptop into one iBook that I can just dock / undock and keep everything on. It's a replacement for my old iBook g3/900.
If you want a desktop PC, the iBook can be used for that, and the parts will cost a fair bit less than a cube. All I need now is to pop a 1GB SODIMM in and I should be set. Total cost for all this stuff, iBook, RAM, dock station, HDD enclosure is just under $1500.