MacMini G4 1.5 GHz, setting the screen resolution in OFW

[Marc Balmer]

I ve got a 1.5 GHz MacMini attached to a 17" IBM TFT panel that has a 
1280x1024 resolution.

How can I set the MacMini's display resolution to 1280x1024 24 bit 
colors in OpenFirmware?

In OFW, 'dev screen show-modes' shows me a list of valid modes, amongst 
them the one I want, but when doing a 'dev screen <n> set-mode' the 
screen only gets garbled.

Any MacMini users around?