You need a low profile or low profile ready graphic card. Since the power supply that comes with the SX2800-01 is only 220w you'd also need to replace it.
This power supply $50
SeaSonic SS-300TFX 300Whttp://www.newegg.com/Product/Product.aspx?Item=N82E16817151070
check measurements herehttp://www.seasonic.com/product/pc_tfx.jsp
The most powerful graphic card available as a low profile ready (you have to change the back plate yourself) is this 9800GT. Some problems are noise and heat.http://www.newegg.com/Product/Product.aspx?Item=N82E16814187061
another good choice, better for noise and heat, but not as high performing is the 9600GThttp://www.newegg.com/Product/Product.aspx?Item=N82E16814162016
The Seasonic 300w can power either card, but you do need to use the included 2-molex to PCIe adapter so check the power supply picture to make sure it has enough connectors for you. With either card you're going to have to watch your temperatures and possibly add fans or replace fans to up your cooling. Either option (there's actually 3 9600GT you could buy) would cost under $200
*EDIT* the 9800GT doesn't come with an adapter, you have to buy onehttp://www.newegg.com/Product/Product.aspx?Item=N82E16812189120