As Strange says play at whatever res and detail level that gives u the best frames.
For a CRT I'd stick with 1024*768 and work on the detail level from there for the smoothest rates, 800*600 is just too low a res for a first person shooter..
My main reason for replying though is put the Hz as high as your monitor will go. 75Hz should be obtainable, it has no performance impact but will stop you going crazy from the flickering you get at lower hertz.