I would highly, highly recommend a card with AT LEAST 4GB of VRAM for 2560x1440 or higher. You'll also want AT LEAST the power of a 780. Thus, from Nvidia, your main options are 970, 980, Titan Black, Titan Z, and 780 6GB. I would probably recommend the 970 or 980 at this time (you might want to wait for the higher VRAM versions). From AMD, I'm unaware of how the non-flagship perform, but the 290X and 295X2 are solid options. The GTX 970 is the best bang for your buck at the moment.