When it comes to overclocking a GPU on your video card the same things come into play with regard to how much performance you can get as you see in overclocking a CPU. The big determining factors are how much power you can get to the card and how cool you can keep the GPU. Keeping the GPU cool often means looking for new cooling solutions.
The catch is that some of the aftermarket cooling solutions for video cards are so large you can't use them if you run multiple video cards inside your computer. Scythe has a new GPU cooling solution that is very thin and still has a big 120mm fan to keep the GPU cool. The solution is called the Setsugen 2 and it has a special connector that will let it work on ATI and NVIDIA cards.
The cooling solution is very flat and thin at only 33mm thick and uses heat pipes that are flush. The fans have an integrated controller that slips into an expansion slot to allow you to turn the fans down when all the cooling performance isn't needed. The cooler measure 176mm x 33mm x 139mm and weighs 435g. The cooler will work with just about every ATI and NVIDIA video card out there. Pricing is unknown at this time.