Far Cry 2
The Dunia Engine was built specifically for Far Cry 2 by the award-winning Ubisoft Montreal development team. It delivers the most realistic destructible environments, amazing special effects such as dynamic fire propagation and storm effects, real-time night-and-day cycle, dynamic music system, non-scripted enemy A.I. and so much more.
FC2 is pretty much playable at all resolutions. A small detail adjustment might be needed at 2560 x 1600, though. The card does fall a bit behind the HD 5850 again, though, at the higher resolution.