if the res manager wont let you change it the only solution is to install it to disk then manually change it if the drivers dont fix the problem.
go into terminal and type..
sudo -i (hit enter)
nano /etc/X11/xorg.conf (make sure you type it exactly like i have it)
you should see this or similar....
xorg.conf said:
Section "Monitor"
Identifier "aticonfig-Monitor[0]"
Option "VendorName" "ATI Proprietary Driver"
Option "ModelName" "Generic Autodetecting Monitor"
Option "DPMS" "true"
EndSection
Section "Device"
Identifier "ATI Technologies, Inc. ATI Default Card"
Driver "vesa"
BusID "PCI:1:0:0"
EndSection
Section "Device"
Identifier "aticonfig-Device[0]"
Driver "fglrx"
Option "VideoOverlay" "on"
Option "OpenGLOverlay" "off"
EndSection
Section "Screen"
Identifier "Default Screen"
Device "ATI Technologies, Inc. ATI Default Card"
Monitor "Generic Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1440x900" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1440x900" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1440x900" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1440x900" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1440x900" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1440x900" "1024x768" "800x600" "640x480"
EndSubSection
EndSection
you may have to manually edit your "modes" by adding the higher resolutions and/or lower your "defaultdepth" to 16.
when your done editing that file i think you press crlt+x then confirm to save.