Overclocking the internal gpu card may be a waste of time and money, but it is something to do for now. When sonnet tech comes out with the PCIe T-bolt adapter this will keep your computer longer and you can update your gpu with ease. also it won't over heat the internals of the iMac
http://www.sonnettech.com/news/nab2011/
Expand Your Adapter Card Possibilities
Sonnets Echo Express PCIe 2.0 Expansion Chassis with Thunderbolt Ports enable you to connect high-performance PCI Express® 2.0 adapter cards to any computer with a Thunderbolt port. Imagine using full-size professional video capture cards, 8Gb Fibre Channel cards, 10-Gigabit Ethernet cards, and RAID controller cards with your new MacBook® Prothe Echo Express expansion chassis makes it possible! Available in two sizes, the standard Echo Express PCIe 2.0 Expansion Chassis with Thunderbolt Ports supports one half-length, double-width, x16 (x4 mode), PCIe 2.0 card, while the XL model supports full-length cards; both models have fans to cool the cards. The standard model includes a built-in 75W power supply, while the XL model includes an integrated 150W power supply with a 75W PCIe power connector. Two Thunderbolt ports support daisy chaining of up to six devices to a single port on the host computer.
Room to ExpandConnect a half-length (full-length in XL model), double-width PCIe 2.0 adapter card to a computer with a Thunderbolt port.
High Bandwidthx16 (x4 mode) 2GB/s PCIe 2.0 slot perfect for special-purpose PCIe cards for video capture, Fibre Channel, 10-Gigabit Ethernet, digital audio, RAID control, multiple screen video, etc.
Power for Powerful CardsIntegrated 75W power supply (150W including one 75W PCIe power connector in XL model).
Two Thunderbolt PortsDaisy chain up to six devices to a single port on the host computer.
some other forums
http://reduser.net/forum/showthread.php?57676-Sonnet-Thunderbolt-PCI-Express-expansion-chassis
http://forum.telestream.net/forum/messageview.aspx?catid=45&threadid=7761