Plenty of posts on that subject m8..did you try a search first?
Anyway at this moment in time Im not sure the make of card makes much difference now though before the ATI users had the blue horizon line syndrome though im sure its fixed now..
1 or 2 Gig? 2gig cards are running better here as a whole..the more VRam seems the better choice but this game is being optimized still so wait untill the next official release (this friday?) comes out and see what people are saying in terms of what card they have and what performance gains they have with the new patch