My vote would be for the new Logitech MX series mouse.
I got the MX300 because when I play games, UT Q3 ect, I use
the mouse in my left hand. They make a MX500, that is right
hand oriented, and a wireless MX700 model.
It cost the same as the Logitech Dual optical, 40bucks, but with
the MX300, I get the ambidexrous mouse, and I think it performs
better than the Dual...
I had a nice 1st generation logitech optical before the MX300, and
the difference is major. Even more precise/fast as the change
from PS2 ball to 1st optical mouse.
Dont install the horrible Logitech mouseware crap however.
Just let windows detect it, then upgrade the driver in device
manager, and browse to the specific hardware drivers on the
CD for whatever OS you are using.
If you do the setup this way, you will get the full 800dpi mouse
functions on either PS2 or USB without all the extra Logitech
software running......