Well, it was over a year ago that I was looking at external graphics adapters for laptops. Fujitsu, as part of their new AMILO Notebook 3650 has released an external GraphicsBooster. The little box sports a 512 Radeon 3870. This is a pretty good idea in many ways, rather than all the weight and heat and size and power and such that a big graphics sub-system needs onboard all the time - having a little box for when you need it has merit.
[Image of the Fujitsu AMILO 3650 and external GraphicBooster via Ubergizmo]
Going by NotebookCheck's rankings for the two cards you would see a BIG improvement.
The built-in shared memory ATI Radeon 3200 scored ok with 1446 in the 3Dmark06
While a Radeon 3870 scored 8421 which is awesome, but I'm not sure if all of that translates through the external connection logic.
MobileTechReview list the specs of things quite nicely and Engadget have their coverage of course as do UberGizmo if you prefer them.