How could I change to 800x600 in text mode ? Which file should I modified ?

How ?

Thanks !

If I understand your question, I think what you need to look at is framebuffering - which may require more than simply editting a single configuration file. Here is a link to start with:
http://www.tldp.org/HOWTO/Framebuffer-HOWTO-5.html

The problem is that I have to run it at 1024x768, and it works well in my old mon. However, now, I use another monitor, and could not enter GNOME, and I believe that it is due to the horizontal and vertical frequency not the same as my old mon. This is becasue the same phenomenon happens in my old mon, and after change to my monitor's horizontal and vertical frequency , it works well in my old mon.

edit the file /etc/X11/xf86config or /etc/X11/xorg.conf andmodify the following section to remove the 1024x768 mode

Section "Screen"
Identifier "Screen0"
Device "Videocard0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1024x768" "800x600" "640x480"
EndSubSection
EndSection


also change the following section for refresh rates

Section "Monitor"
Identifier "Monitor0"
VendorName "Monitor Vendor"
ModelName "LCD Panel 1024x768"
HorizSync 31.5 - 48.5
VertRefresh 40.0 - 70.0
Option "dpms"
EndSection