thats a pretty good budget
you want one that has a low response time; somewhere in the 2ms - 8ms range. the lower the better. You also would want a nice resolution, at least 1280 X 1024 if not 1600 X 1200, but the higher the resolution the less fps you get on high settings, unless you have a 880GTX/GTS. you also would want a high contrast ratio to get a good high definition picture. you would want a good brightness too so the colors look true and accurate. widescreen is your choice but i wouldnt recommend it because not all game support the widescreen resolutions.
19"-Viewsonic~nice response time of 2ms, contrast ratio is lower than other at 650:1 and brightness is lower than other at 270 cd/m2, good resolution of 1280 X 1024 which is getting to be standard
http://www.newegg.com/Product/Product.asp?Item=N82E16824116375
20.1"-Samsung~nice response time of 5ms(for big size), nice contrast ratio of 800:1, nice brightness of 300 cd/m2, nice resolution of 1600 X 1200, lots of colors at 16.7 million
http://www.newegg.com/Product/Product.asp?Item=N82E16824001226
22" WS-Sceptre~nice response time of 5ms, nice contrast ration of 1200:1, nice brightness at 300 cd/m2, nice resolution of 1680 X 1050, lots of colors at 16.7 million(its a nice widescreen but games might not have this resolution)
http://www.newegg.com/Product/Product.asp?Item=N82E16824112005
those would be the ones i would choose from