edit your xorg.conf , which is in /etc/X11/ , and scroll down to the screen section and look for something like this...
Code:
Section "Screen"
Identifier "Screen 1"
Device "ATI(RadeonGeneric)"
Monitor "GatewayVX700"
DefaultDepth 24
Subsection "Display"
Depth 8
Modes "1024x768"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 16
Modes "1024x768"
ViewPort 0 0
EndSubsection
Subsection "Display"
Depth 24
Modes "1024x768"
ViewPort 0 0
EndSubsection
EndSection
simply add the desired resolution to each of the displays, save , and restart x, and then you should be able to select your desired resolution .