The quality of monitors has got to the stage where the performance between them has significantly narrowed compared to what it was. First you need to decide on the approximate size of monitor and again that depends on what you will be using your system for and as previously mentioned if you want to have one or two on your system.

The main thing to worry about is to get the connections correct as there are now an increasing number of graphics cards and monitors that only support the HDMI sockets which is not much fun if your graphics card has the standard monitor output but your monitor is expecting HDMI. There are loads of good monitor manufacturers but my favourite in terms of cost verses performance are Asus, Acer, BenQ and AOC.