Upgrade Alienware m17x R3 Graphics Card

I dont understand if I can upgrade my alienware m17x r3 graphics card

I have got the NVIDIA GeForce GTX 460M (i think i have 1.5 gb of it, if not, 2) and would like to update it to a very high end graphics card.

I have got a few questions:

1. can I change the video card?

2. if i can, would i be able to change the type of video card (for example, change to AMD)?

3. what kind of video cards can i buy (does the m17x only fit in a single card, not dual or does it need to be a mobile version like the GTX 660M) and which would you recommend?

Thanks :D