Why?
The purpose of a solution like this would be met best in this scenario as Kenny pointed out: You buy a small portable notebook (13-14" screen) packing a C2D, 2GB of ram, GMA950, and whatever else you want. Basically, a configuration that can run around and get the 4-6 hours battery life as mentioned. You get home from school/work/gf's/whatever and go to your desk. You set your laptop down onto its dock to the side, and plug in the graphics card to its port. You're now sitting at your standard kb and mouse, your nice gaming widescreen lcd, and surround sound setup ready to play Crysis on whatever DX10 card you have in the graphics bay. Maximum mobility on the run, maximum power at home. And with your 2GB of ram and fast C2D, you can still easily work on the run and at home. Also removes the heat issue of vid cards from affecting other parts of the lappy.
This issue at hand here is that an interface needs to be standardized. Something like a PCIe External x8 slot minimum available externally. They ahve eSATA, and the X34 slot that is a 1x PCIe link. x8 link is needed minimum, if not a full x16 bandwidth slot. This is what this all hinges on.