IMHO, looking for a DVI connection is a good idea. If it's DVI-I it'll also support VGA with the right adapter.
http://en.wikipedia.org/wiki/Digital_Visual_Interface
Consider getting a display that'll rotate into portrait mode. I find this very useful.
Some also have video input too that might be handy if you have a games machine or an STB you might want to connect.
Aria always used to be a good place to look for monitors:
http://www.aria.co.uk/