If you don't want to mess around with arcane incantations inside xorg.conf, you can use the graphical arandr tool:

sudo apt-get install arandr
arandr

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If all your monitors are stacked in the same space, the space is not wide enough and you can't really move them, you need to switch to a proprietary driver. If it works with the open source driver for you, do not bother using another one.

For ATI cards:

• install fglrx and fglrx-amdcccle
• reboot
• run the ATI configuration tool (amdcccle; if the binary is unavailable for you, locate and run it directly -- it's probably at /usr/lib/fglrx/bin/amdcccle).
• Configure your monitors there and restart X (or your whole machine).

If your layout is still messed up (sometimes happens) you can at least use arandr now.

For nvidia cards:

Probably something similar (we don't have any on our office machines).