I am not able to get max resolution of 2560 x 1440 on Dell P2418D monitor. Graphic card I am using is GV-R645SL-1GI and drivers are up to date. I used HDMI cable to connect those two but I don't know cable specs. Wondering if this issue is because of that cable or something else?
2048x1080 is actually a lower resolution than 1920x1200 in terms of pixels/bandwidth. To get the full 2560x1440 from your monitor, try a dual-link DVI-to-HDMI adapter.
The P2418D monitor on a hardware level supports 2560x1440 via its DP and HDMI in ports. See pages 8, 14, 15, 47 of the User's Guide. So the fault has to be the video card or its drivers, not the P2418D.
As swamped stated, the low end AMD Radeon HD 6450 HDMI out port maximum resolution is 1920x1200. I cannot find any adapter that can convert from HDMI to DVI at 2560x1440. All I can find only do 1920x1080. My advice, get a better video card with a full size DP out port.