Everything is relative. If you have very, very strong screen resolution, maybe you need even more DPI. But for the most part, if you've got a regular, modern monitor, you'll probably want a lower DPI to have more precision. I use 1600 DPI and I only move my mouse within a 1 inch^2 square to hit all four corners of my screen. In general, it seems that most good mouse-only players (players who play solely with the mouse and not with the keyboard) play with 400 DPI and need a considerable amount of physical movement to move their cursor. Mouse+keyboard players tend to use anything from 400DPI to 3600DPI. I think the most common mouse sensitivity in the higher ranks is 800DPI but I'm not certain. The currently highest ranked mouse player uses 2000DPI.
Don't take any of that to be fact, but that's what I seem to see and I hope it helps =)
Don't take any of that to be fact, but that's what I seem to see and I hope it helps =)