Tidus brought up a good point. Your CPU would be the bottleneck at this point. 1920x1200 is a lot to ask for, so your processor will need to be overclocked to help a bit. Although the 1GB version of your graphics card will do a lot to help you as well (since 1920x1200 would command your graphics card a lot more).
But right now, trying overclocking your CPU and see what you get.
Just a thought...