Falcon Northwest's custom FragBox gaming computer manages to stuff a modern processor, two graphics cards and 24 gigs of RAM into a case the size of a largish shoebox. It's even got a handle, so you can hypothetically carry around with you.
It's always tough to decide whether to buy a laptop or a desktop. Do you go more expensive and less powerful and portable, or slightly cheaper and beastly and hard to lug around? The FragBox is a compromise, sort of, putting all the guts and glory of a high-end gaming PC into a 15 inch long, eight inch tall case that weighs a mere 25 pounds including a carrying handle. I'm not sure I'd call it portable, but I would call it more portable than most desktops.
The FragBox doesn't skimp on the specs, either, finding some way of holding a staggering 24 gigs of RAM, dual Radeon 6970s, an Intel Core i7 Extreme processor, and a 256 gig SSD, all of which only cost me about $5,600 when I configured my fantasy system just now. Lesser (but still respectable) systems start at a mere $1,700, and how could you not paint it orange for just $400 more.