I'm sure this has been covered many times before, but we can cover it again.
You need to modify your X11 configuration file. It is probably located at /etc/X11/xorg.conf or /etc/X11R6/xorg.conf. You will find a section called "Screen." There should be a line called "DefaultDepth" which tells which color depth to try first. Under the subsection with the same depth as your default depth (probably 24), you need to set the first resolution on the "Modes" line to the resolution you want.
To modify the horizontal sync rate and vertical refresh rate, see the section called "Monitor." There are two lines you will be interested in here, "HorizSync" and "VertRefresh." Be careful when changing these. X11 does not check to see if your monitor is actually capable of running what the config says, so you could seriously damage your monitor.
Here is a sample from my xorg.conf:
VendorName "Monitor Vendor"
ModelName "LCD Panel 1024x768"
HorizSync 31.5 - 48.5
VertRefresh 40.0 - 70.0
Viewport 0 0
Modes "1024x768" "800x600" "640x480"
I hope this helps. Good luck, and be careful.