Give XF86Setup a try (if you have it installed.) I never had much luck with
xf86config or Xconfigurator myself, but I usually got a working config file
out of XF86Setup. THe other bit of advice I'll give is to know your monitors
hariz and vert refresh rates. If you configure X with those instead of the
safe defaults you can push your card/monitor to the max without wierd X is
kinda working at that res situations...if XF86Setup doesn't do the trick,
pop us off another message. 

--
Andy Zbikowski, Sys Admin   | (WEB) http://www.ltiflex.com
LTI Flexible Products, Inc. | (PH)  763-428-9119 (EX) 132
21801 Industrial Blvd       | (FX)  763-428-9126
Rogers, MN  55374           | (PCS) 612-306-6055