If Shuttle can pack a quad core processor, 8 gigs of RAM, 3 x 3.5" hard drives, a dvd burner and have room for a dual-slot NVidia 8800 GTX into a box this size, Apple should be able to do it too:
It's the headless iMac / Cube 2.0 / Mac Mini Extreme / Mac Pro Nano.
There's a good overview/review of one of the Shuttle PC's here:
http://www.trustedreviews.com/pcs/review/2007/05/30/Shuttle-SD39P2-Barebone/p4
and of course Shuttle's website is:
http://www.shuttle.com/

It's the headless iMac / Cube 2.0 / Mac Mini Extreme / Mac Pro Nano.
There's a good overview/review of one of the Shuttle PC's here:
http://www.trustedreviews.com/pcs/review/2007/05/30/Shuttle-SD39P2-Barebone/p4
and of course Shuttle's website is:
http://www.shuttle.com/