Normal consumer GPUs will not work, unfortunately. The slot is meant for adding an additional port to the existing on-board graphics. You can get a card to run dual graphics(the existing and the expansion card port), but I hope you don't want to do any gaming with it.
You want to get this:http://www.newegg.com/Product/Product.aspx?Item=N82E16815990004
It's not available at the Egg, but you may be able to find it on eBay or something.