This is what I use in xorg.conf on Slackware for my USB mouse.
Code:
# Core Pointer's InputDevice section
# **********************************************************************
Section "InputDevice"
# Identifier and driver
Identifier "Mouse1"
Driver "mouse"
Option "Protocol" "IMPS/2"
# Option "Device" "/dev/usb"
Option "Device" "/dev/mouse"
Option "ZAxisMapping" "4 5"
Option "Resolution" "300"