Almost as a rule, you can't upgrade laptop graphics.
The only option that I'm aware of is the ViDock: http://www.villagetronic.com/vidock2/index.html
Tom's did an article on this product in 2008. My understanding is that the Expresscard interface speed has doubled since then; 3D performance should be better than Tom's found in '08.
Unfortunately, your notebook has the older PC card slot, so this isn't even an option.
Looks like you're out of luck, sorry.