Using a Mac Mini at 1920x1200 I got a Mac Mini at about summer in 2011 and I'm looking to use it along with my main PC using a Dell Ultrasharp U2412M monitor. The monitor's DVI is used by my main PC, so I picked up a Mini DisplayPort to VGA adapter (at the time I didn't actually know the monitor had a DisplayPort on it, but I'd still need a Mini DisplayPort -> DisplayPort adapter) and tried it.

But I can only set the Mac Mini's resolution to 1920x1080 in System Preferences.

I tried modifying my com.apple.Boot.plist file in Library/Preferences/SystemConfiguration, and it currently looks like this :

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
        &lt;key&gt;Kernel Flags&lt;/key&gt;
        &lt;string&gt;&lt;/string&gt;
        &lt;key&gt;Graphics Mode&lt;/key&gt;
        &lt;string&gt;1920x1200x32&lt;/string&gt;
&lt;/dict&gt;
&lt;/plist&gt;


That doesn't seem to have helped. Any further ideas?
Tha monitor has a DisplayPort connectiion as well as DVI. I'd suggest getting a mini DisplayPort to DisplayPort cable. That way you get the benefit of a digital connection and proper resolution. VGA isn't really designed for resolutions that high — it's possible, but often shaky.