If you could post your XF86Config file, (or XF86Config-4 depending on which you're using), it would be beneficial.
In general, a mode line like:
# 1024x768 @ 60 Hz, 48.4 kHz hsync
Modeline "1024x768" 65 1024 1032 1176 1344
768 771 777 806 -hsync -vsync
should probably work. With the Screen setting for generic LCD.
Looking back through my notes, I see that my method to discover the real refresh rate my win98 box was using was to switch resolutions on the fly, and jot down the refresh and horizontal sweep frequencies as my multisync monitor displayed them. (Some multisync monitors will flash their video modes on the screen as they decode the input from a video source, your monitor mayhap does not.)
hth
Mark