need to upgrade the radeon hd 5870 in Alienware M17x R2

The reason I think I need to replace the graphics card is that the display driver quits working when a video is played and Windows Media Player said that a video could not be played because there is something wrong with the video card.  Here is what I know about the computer...

This computer was originally purchased in 2010. 

It has dual ATI Mobility Radeon HD 5870 with a total memory of 4851mb.

Processor: Intel Core i7 CPU  x 940 @ 2.13 GHz

RAM: 8 GB

System: Windows 7 64-bit

The repair manual shows that both primary and secondary cards can be replaced, but I have no idea what to replace them with and the parts list on dell does not list a graphics card.  Any help in this area is greatly appreciated!