I recently bought a Samsung monitor that has a resolution of 2048x1152. I also got a Powercolor HD3650 video card.
After tinkering for hours, I finally got hardware acceleration to work when I adjusted my resolution to 1920x1080. DXVA is definitely being used because my CPU usage went down a lot. My problem is that @ 1920x1080 the text is blurry when I'm not watching movies, and I want to make use of the high resolution.
I have the exact same monitor and I have no problems switching between the resolutions , thou I'm constantly using 2048x1152.
So try switching from 1920x1080 to 2000000000000 res. and you'll have no more issues. I'm watching movies at the monitor's native resolution and have no issues.