Have you put the proper values for the Horizontal and Vertical refresh rates for your monitor in the configuration file most times in my experience the configuration program(s) usually setup very conservative rates. And you should look in the log file of X everything thing you need should be in the file if your monitor is DDC compatible and of course you are loading the ddc module in the config file when starting X, on my system the file name is /var/log/XFree86.0.log. If you cannot get it going can you post the config file you are using and the log file. To answer your question at the bottom X will use the highest refresh rate possible according to what the monitor tells it from the DDC information, the abilities of the driver used and the modes supported by the video card.
Edit: An example of what the section looks like in my config.
# KDS Visual Sensations VS 190i
#DisplaySize 350 265 # mm
Option "DPMS" "true"
HorizSync 30 - 95 # DDC-probed
VertRefresh 50 - 120 # DDC-probed