OK. I installed RedHat 9 on a different partition and then installed the nividia drivers there. Not only was I able to get 1600x1200, but I was able to get 1920x1440!
So, there is something wrong with my X installation. I'm going to recompile it with less aggressive optimization settings and see what that does. I'll also look through the RedHat patches applied to the stock x 4.3.0 package and see if that turns up anything. I'll post the results when I am finished.