Upgrading Alienware x51 Video Card

Good Evening,

I am looking to upgrade my Ailenware x51 Desktop Graphics card.  Currently I have GeForce GT 640 and I'm looking to upgrade to the GeForce GTX 750 Ti or any card that is a workable upgrade.  I want to make sure that I upgrade using the right card because I don't want to damage my computer.  My Service Tag is Service tag removed per privacy policy>.  I'm not sure if I have an x51 R1 or R2, even R3 if that exsists.  Could someone please provide a list of acceptable Video Cards to use in my system?