Vostro 3910 HDMI lower Resolution than Displayport

I've just installed a new Dell Vostro 3910. It has both a display port and a HDMI. I've got two identical Dell U2515H monitors, one plugged into the Displayport, one into HDMI. 

Displayport monitor max resolution is 2560x1440
HDMI monitor max resolution is 2048x1152

2560x1440 is the native resolution of both of these monitors. The screen definition/quality is noticeably lower on the HDMI monitor. 

So how do I get the 2560x1440 on both monitors? Time for a new graphics card or is there a better solution? Thanks!