You can't do it yet... the bandwidth is still too limited on those Thunderbolt chassis for high end graphics. You can put a 10GB Ethernet card, an Esata card, or other peripherals in it however.So, at least in theory, one could add a graphics card with Sonnet's Expansion Chassis? Didn't find anything about the support in their web site though.
There's one chassis that runs at twice the speed of the sonnet currently available, but unfortunately it costs a grand.
http://www.magma.com/thunderbolt
People are working on graphics solutions though:
http://www.youtube.com/watch?v=-AYypyF1SRg