MX510, but I may get the 518. The 1600 dpi is definitely appealing.
As a hardcore FPS gamer, I use low sensitivity together with a nice 11"x15" mousepad, so the data transfer rate for the mouse is definitely the most important parameter, or has been until now. The 510 was the first mouse that I came across that wouldn't skip during long-distance flicks (when it takes you 6 inches to make a 180, you tend to notice skipping). However, the resolution of the 510 is really not that great. It doesn't matter for low sens, but it would matter for photoshop or for high sens gaming.
Wireless mice are useless for games because of bandwidth issues and time lag. The bandwidth is actually fine for some of the newer wireless mice and they won't skip too badly, but the response lag is definitely noticeable at higher skill levels. The cord bothers me, but I weigh it down with the keyboard, and it doesn't get in the way.