DirectX 11 class graphics card performance
Here we are at the most common used resolution for the Radeon/GeForce gaming class, 1920x1080/1200 are the most preferred monitor resolutions this year and age.
The GeForce GTX 560 Ti is still sufficient, your best next model is the GeForce GTX 570 or the owners with a GTX 480 are laughing hard as the now two year old series is still a perfect card to do the job.
AMD Radeon wise well, the cards are performing really well overall. Even a Radeon HD 6870 will be very suitable.
For bragging sake... look at the all new Radeon HD 7970 go by the way, it is capable of battling with the multi-GPU based graphics cards extremely well. The AMD Radeon series are doing really well with this title.
Once the Radeon HD 7990 will be released performance would be ridiculous really.
And last but not least the mac-daddy of single monitor resolutions, on a 30" 2560x1600 pixels monitor, this resolution will require a spicy graphics card as it renders over 4 Mpixels per rendered frame (!).
A GeForce GTX 580 as recommendation would be valid, but again... look at the new Radeon HD 7970 go.
Next to these two cards only dual-GPU based solution can really deal with it. On AMDs side the new Radeon HD 5970 and 6990 manage really well here, very much recommended. On NVIDIA's side, yep... the GTX 590.