I have two different methods you can try.
1. Modify monitors.conf
I'll use kingbeowulf's monitors.conf as the example. In the display subsection:
Modes "1600x1200" "1280x1024" "1024x768" "800x600" "640x480"
If you want 1024x768 to be default you move that to the front.
Modes "1024x768" "1600x1200" "1280x1024" "800x600" "640x480"
See if that works. This method has problems with the nvidia driver (I don't know whether the same problems exist on other drivers), which leads me to the second method (it may also be simpler).
2. Add a xrandr line into the appropriate startup script.
If you login at the console you change xinitrc.
If you login via the KDE login manager I think you change /etc/kde4/kdm/Xsetup (I don't use KDE, so I may be wrong)
Add the following line into the appropriate file:
xrandr --output output_name --mode 1024x768
Replace output_name for whatever it is called on your system, if you don't know what it's called run xrandr. One of the lines it outputs in my system is
DVI-I-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 598mm x 336mm
In my case, output_name is DVI-I-1.
This worked for me using the nvidia driver and should also work fine with other drivers as well, so I would probably recommend this method first.