I don't like Dell computers, but I only hear good things about their LCD monitors. For gaming, get a 17". The reason is, you want to run the LCD at its native resolution, and 17" inch's native resolution is 1280x1024 I believe. Any higher than that in games (19" monitors or larger) will be more stressful on your computer, and cause lower framerates. LCD's visual quality decreases when ran at non-native resolutions.
Edit: Well.. if you have a real good computer, then maybe go for 19". But just remember, as newer games come out, and your computer gets older, having to run them at high resolutions may kill your performance and make it essential to upgrade more often. Just my 2 cents.