I fore-see it as having it's own mainboard. These mainboards would come in several flavors.
Single GPU with special sockets or Dual GPU. The GPU's could technically be drop in. After market coolers available. There would also be a socket for a PPU, which could also be drop in. There would be say 2 or more RAMBUS XDR slots onboard so that you may upgrade as you please. Flash-Rom chips onboard for drivers.
Cases could be fully moddible or buy them as funky as you wish. Power Supplies would be after market or come with the cases.
Simply connect the unit to the PCIexpress 16X slot (via) a special rear panel adapter... plug in your monitor, and you're gaming like never before....
There are several things you'd need:
1.) A Mainboard manufacturer - like (DFI) - Lot's of R&D
2.) ATI or NVIDIA to make drop-in GPU's.
3.) Ageia to make Drop-in PPU's
4.) Flash-Rom or other component to store drivers / bios
5.) Special PSU's - PCP&C - OCZ - ANTEC ?
6.) Small Formfactor Cases with excellent cooling - Thermaltake?
7.) After market coolers - air, water, etc...
8.) Compatible RAMBUS modules. 256MB, 512MB, 1024MB
Think about the next generation Consoles right.... They are going to be extremely powerful and cost under $500.00. All of these things i'm talking about are more or less inside of a next generation console. Why not kick those consoles up a notch... well, they won't be consoles... they'll be "External Graphical Systems" or "XGS".
Everyone can dream can't they? lol