[citation][nom]randomizer[/nom]An MS Intellimouse is fine for just about anything[/citation]
It's less to do with speed, I think, and more of sensitivity at larger screens. I used to be one of those "it's not the tool you use, it's how good you are", but now that I game on two different machines, I can tell there's a difference. On one machine, with 1800DPI, I've got the sensitivity in some games doubled or tripled the standard. Then on my razer I have upstairs (diamondback, I think - I can't remember), I leave it on the standard sensitivity or even half what's given. The thing is isn't how fast - i.e. one both machines, it's the same speed; if I move 2 inches on the mouse pod, I've gone at least 360deg in most FPS's I play.
But what's noticeable - say I'm sniping on the pc that I've got 1800DPI - I'll find that distance shooting can suffer. I can snap to target quick enough, but sometimes the target lies between points. I.e. I do slight left, and I'm over right shoulder. I nudge right to make a headshot, and the reticule skips past the head over left shoulder. The lack of DPI is a lack of sensitivity - at far distances on high res games, this means a lack of pixel perfect precision. Same shot on my 3600DPI (or whatever it is) mouse, and smooth as butter. Same range, same weapon, but now I can nudge pixel by pixel, instead of skipping. So I can make that headshot without having to physically move my character left or right.