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That's a mini DVI port.

Depending on your monitor you will need a Mini DVI - DVI or a Mini DVI -VGA adaptor (about $19 from Apple) and the relevant cable (DVI or VGA) to connect from the adaptor to the monitor.

If your monitor has DVI, use that as it's better.
 
It would all depend on the display you are connecting it too. If it has DVI then thats the one to use. Im on an LCD tv so I have DVI to HDMI.
 
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