The only commercial product is the vidock but expresscard creates a big bottleneck. You'll have to wait for usb3 or thunderbolt to become more popular for other solutions (although these connections still bottleneck). Or you could look up current diy solutions.
It's been talked about for a few years and a prototype has been developed by Asus called the XG station which used a Geforce 7900GT I think back in late 2006 or early 2007. But there was no follow through. I think it was going to be priced at $400 - $500.
These are only external graphic card cases. You still need to add the cost of the video card. It doesn't specify which USB version is supported, so the natural assumption is USB v2.0. It's not as fast as an actual PCI-e interface, but if you want an external video card, then this is your only solution.