Depending on your distro, there is usually a GUI to modify your display settings. But if you can't find it, you can just manually edit the /etc/X11/XF86Config file:
Just open up the file in a text editor and look for the "Screens" Subsection.
Code:
Section "Screen"
Identifier "Screen0"
Device "RIVA TNT2"
Monitor "Monitor0"
DefaultDepth 16 <------ Change here
SubSection "Display"
Depth 16 <------ and here to new settings
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection
Then just save that file and restart X.