HOWTO use an external display

-- lspci output --
01:00.0 VGA compatible controller: ATI Technologies Inc Mobility Radeon HD 3650
--

# pacman -S xf86-video-radeonhd

-- xorg.conf --
...
Section "Module"
...
Load "dri"
...
EndSection
...
Section "Device"
Identifier "Card0"
Driver "radeonhd"
VendorName "All"
BoardName "All"
Option "AGPMode" "4"
Option "ColorTiling" "on"
Option "AccelMethod" "EXA"
Option "EnablePageFlip" "on"
Option "AGPFastWrite" "yes"
EndSection
...
Section "DRI"
Mode 0666
EndSection
--

# /etc/rc.d/gdm restart

Open a new Terminal in X

$ xrandr -q
Screen 0: minimum 320 x 200, current 1600 x 2000, maximum 2560 x 2560
VGA_1 connected 1600x1200+0+800 408mm x 306mm
...
...
PANEL connected 1280x800+0+0 331mm x 207mm
...
...
$ xrandr --output VGA_1 --mode 1600x1200 --below PANEL

Instead of --below you can use --right-of --above --left-of