My PC runs at a 1440x900 resolution. In the Dark Souls options page, I do not see that resolution listed.
I tried picking the next resolution down, but it creates black lines at the top and bottom of my game.
How can I play with a full resolution?
A player named Durante has created a mod that will allow you to set a higher/custom resolution. You can download it from here, and follow the instructions in the readme file.
This mod works by intercepting calls to D3D9.dll (Direct3D), changing the resolution in the call, and passing it on to the real D3D9.dll.