Look for low response time, high contrast ratio, and good reviews. 17 Inch ones have a better pixel pitch so they scale to non native resolutions better then 19 inchers. I personally hate widescreen but that's just personal preference.
Don't be fooled into thinking dell is the best for monitors, there are other companies out there that I would much rather buy a monitor from. Samsung, viewsonic, hyundai and more. Personally I have a Hyundai L72s monitor, it's great for games.
look for a response time of 8 ms or less.