I thought HDMI cables couldn't support 4k at 60fps, or am i behind with the times?
HDMI 2.0 does. But the specific Dell monitor above has 2 display ports.
Either way, 5120 x 2880 is not that difficult tbh. Without AA a CF 290X/295X2/ SLI Titan/SLI GTX780 6GB, might do it.
However, we are talking here about almost double the amount of pixels between 4K and 5120x2880 on a 27" screen, and even 4K at 28" looks bit daft