Probably looking at a GTX570/6970 like Monsta said, or you could go for a GTX580, or a 7970. All up to budget.
i can max out ultra settings(motion blur off)on a gtx 580 and fps never go below35~40 in heavy battles.otherwise above 60.a 570 will do fine but not everything at max.fps will drop below 30 and game will become choppy.what's your budget ?
gtx 560ti 448 cores graphic card will be an good one and its nearly comparable towards gtx 570.:)
Other options are crossfire HD6950s (around power of 2 GTX570s)
Crossfire 7970. Or 1 7970.
what? a single 7970 maxes out the game.crossfire will destroy it!