Video card for XPS 710

I need to replace the video card in an XPS 710. I'm almost 100% sure that the card in it has gone bad - the card in it is 256MB nVidia GeForce 8600 GTS. It is running Windows XP and has 2 GB DDR2 SDRAM in it. It is mainly used for office applications and some light gaming. I've looked for replacement cards and am totally confused as to what card will fit. 

Could you suggest a card or give me the specs for what I need to buy since I will be purchasing it online most likely.

Thank you!