I can't give you any specific recommendations as your budget and brand preference will vary but you might find it useful to know what the GPU (GeForce GT 620) in that machine can handle.
Display Support:
Multi Monitor: Yes
Maximum Digital Resolution: 2560x1600
Maximum VGA Resolution: 2048x1536
HDCP: Yes
Standard Display Connectors: Dual Link DVI-I, HDMI, VGA
http://www.geforce.com/hardware/desktop-gpus/geforce-gt-620/specifications
That said, I personally ususally shop for the highest resolution LCD monitor I can afford that my machine can handle, in this case 2560x1600, and I'd lower it if I had to. They say LCD's don't look as good on anything other than their native resolution. Most don't look "horrible"so this isn't a concern for me. If 2560x1600 is too much, shop for something that's suitable to your vision so you don't have to change it and sacrifice quality.