I tried to let the kernel do it but can never get it to work properly. You will have to rebuild the kernel to get this to work. If you do not know how to build a kernel, you may want to learn it is a lot easier now. Before building kernel, you may need the maestro3 sound driver.
XF86Config using XFree86 18.104.22.168
I tried using others on the net and none of them worked. I have also tried using the xfconfig. You will notice I am not using the modelines. This is not necessary with XFree86 4.x. So your mileage may vary. I can get 1024x768 and 1400x1050 but not the resolutions in between. Here is mine.