Avec ça, je fais rouler mon MX500 :

Section "InputDevice"
    Identifier  "SourisUSB"
    Driver      "mouse"
    Option      "Device"        "/dev/input/mice"
    Option      "Protocol"      "ExplorerPS/2"
    Option      "Buttons"       "7"
    Option      "ZAxisMapping"  "4 5"
    Option      "SendCoreEvents" "true"
    Option      "Resolution"    "100"
    Option      "SampleRate"    "800" 
EndSection