How to change what my mouse buttons do?

[Chris G]

I have a Logitech trackball which X (on xubuntu 8.10) detects and
configures quite successfully:-

    (II) config/hal: Adding input device Logitech USB Trackball
    (**) Logitech USB Trackball: always reports core events
    (**) Logitech USB Trackball: Device: "/dev/input/event2"
    (II) Logitech USB Trackball: Found x and y relative axes
    (II) Logitech USB Trackball: Found 5 mouse buttons
    (II) Logitech USB Trackball: Configuring as mouse
    (II) XINPUT: Adding extended input device "Logitech USB Trackball" (type: MOUSE)
    (**) Logitech USB Trackball: YAxisMapping: buttons 4 and 5
    (**) Logitech USB Trackball: EmulateWheelButton: 4, EmulateWheelInertia: 10, Emu lateWheelTimeout: 200

However I really want the two extra buttons on my trackball to be a
Middle Button (button 2) and a double click of Button 1 rather than
their attempts at emulating a wheel (which I don't know how to work
anyway). 


So how do I change what the buttons do?  Any information or pointers
to web pages would be gratefully received.

(I also don't quite understand why it says "Found 5 mouse buttons",
there are only four)

-- 
Chris Green