Look at the back of your case.....does the card block one or two slots at the back of the case. Another way to think of it is if you put 2 screws in to hold it in place, it's a dual slot card, 1 screw = single slot.
The difference is, a single slot card exhausts heat into the case . A dual slot card exhausts hot air outside the case thru an external grille which takes up the extra slot..
This is a dual slot cardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814150357
This is a single slot abomination http://www.newegg.com/Product/Product.aspx?Item=N82E16814125266