I finally had a chance to review this guide from my home system that has my Ti4600. Following this, 1024x768 with Quincux in D3D runs fabulously and looks no worse than before, from what I can see. I might play around with a couple more settings, but I think we might have a winner here.
Side note w/regard to mouse lag. You can set it to false in the UT2003.ini file, but if you have RivaTuner in the D3D settings, under vsync, set your prerender limit to 2. That should reduce any mouse lag significantly while still allowing you to have the better performance that setting mouse lag to false in the .ini file provides.