It may not handle it correctly. You should guarantee that you have a good mode by using a modeline. 'gtf' (shipped with X 4.3) will generate a modeline for you. FWIW, i've got an 8500 working with 1920x1200 without problems using:
Modeline "1920x1200" 162 1920 2020 2052 2184
1200 1202 1208 1235 +Hsync +Vsync
However, you should make sure that this modeline is appropriate for your system before trying it.