With the help of "logitech_applet", I am able to set the resolution of the mouse from 400 to 800dpi. Since I don't want to do this everytime I plug in the mouse by hand, an udev-rule comes to help: Just create the file /etc/udev/rules.d/10-user.rules and put in the following line (yes, it's one single line):

SYSFS{manufacturer}=="Logitech", SYSFS{product}=="USB-PS/2 Optical Mouse",
 NAME="input/%k", MODE="0644", RUN+="/usr/sbin/logitech_applet -s 800"

I found this at the Gentoo udev wiki entry.