Alienware video cards

[Proagie]

I have an alienware m15x, I purchased it in 2008. Specs are as follows....
Video/Graphics Card: NVIDIA® GeForce® Series - 512MB NVIDIA® GeForce® 8700M GT Processor: Intel® Core™ 2 Extreme X9000 2.8GHz (6MB Cache 800MHz FSB)
Memory: 4gb
System Drive: 320GB 7,200RPM (16MB Cache) w/ Free Fall Protection
Optical Drives : 8x Dual Layer Burner (DVD±RW, CD-RW)
Wireless: Internal Intel® Wireless 4965 a/b/g/Draft-N Mini-Card
Sound Card : Internal High-Definition Audio with surround sound

I would like to upgrade the video card on my own. I need a list of compatible cards. The website no longer shows specs for this version of the m15x, so I would need someone who is knowledgable about this to give me some info.

 

[100100]

There are various graphics cards which can fit into it, such as the GTX280M.
The problem lies in installing in and the price. I saw a GTX280M selling for around 500$.