I have a HP elite hpe-345sc with geforce gtx 260 1.8gb and a widescreen about 22-23 inches. The geforce card supports 2560x1600 at 60 Hz with hdmi and my max possible resolution on my screen is about 1900x1080 I think or something close to that.
I run everything on high settings in diablo 3 and with AA on, but the graphic is kind of rough, compared to what I have seen on smaller screens (and lesser powerfull pc and laptops), where the graphic is much more "round".
Is this a problem caused because my screen is to large or because Diablo doesnt support it or is it the geforce card??
make sure that your game is set to run at 1920x1080 actually (i've seen many people set their game to low resolution, have the gpu scale it to full screen after rendering and then they wonder why the graphics don't look so hot)
on top of that I can tell you that D3 graphics are not that sophisticated