U2515H Monitor HDMI output unable to set to 2560x1440 Resolution

Hi,

I just bought two u2515h monitor from Dell.

I have a PC with ATI Radeon HD 5800 graphic card, which have 1 HDMI port, 1 DP port, and 2 DVI port.

So i have no option to use two DP, i connected the first monitor(u2515h) with mDP to DP cable which included in the package of the monitor. I set the resolution to 2560X1440 without any problem with DP Connection.

However, when i try to setup the second monitor(u2515h)  with HDMI to HDMI connection, i am only able to set maximum resolution of 2048X1152. There are black space around the screen and the text look blurry, so i go into the CCC of ATI to scale up the size to 0% after doing some research online. It only enlarge the desktop. The monitor is still not able to use it's max resolution(2560X1440) via HDMI to HDMI connection, and the text are still blurry.

I am not sure what is the problem here, so i figure to ask at the forum to see what should i do. Following are some thoughts that i think could solve the problem, please let me know how to solve this issue, as the lower resolution with blurry text really annoy me for the second monitor.

Specification:
- OS: Windows 7 64bit
- Motherboard: P7P55D PRO
- Graphic Card: ATI Radeon HD 5800 
- Monitor Setup: extend monitor Setup with both u2515h
(1st with [Monitor]mDP to DP[PC] connection: max res 2560x1440)
(2nd with [Monitor]HDMI to HDMI[PC] connection: max rest 2048x1152)

Problem:
- Unable to set to maximum resolution for the second monitor with HDMI connection.
- Blurry text with the second monitor

Possible Solution:
- I am not sure if the HDMI port from u2515h support 2560x1440 resolution, or it's the cable problem?
- Should i purchase a mDP to HDMI cable so that the monitor can connect to my graphic card HDMI port? (If the problem is the HDMI are not support to set max res)
- If the worst case, i have to replace my graphic card? If yes, which option can i go for? i have a pretty old motherboard P7P55D PRO

Please help me to solve this problem, i really unsure what should i do, i better get some advice before i purchase anything useless to solve this problem.

Thank you very much!