Alienware M17x R4 Graphics Card upgrade?

Hey everyone,
So last Christmas I bought an Alienware M17x R4 with a GeForce GTX 660M. 
I was wondering if I could upgrade my graphics card? Everything I have found has said yes that I can, however I was wondering how I could
go about getting a new one? From what I found I believe I can upgrade to a GTX 680M - Can anyone confirm this?
I know how to install the new Graphics card, I guess my real question is how can I go about getting the new one and from where? Also what will
the pricing look like? Will I need a new heat sink and thermal paste?

Thank!
-Robert