Thanks to everyone for their help on this. It seems that the only way to change my X resolution is to do so in MacOS before I boot. fbset has parameters to set X resolution - but then if I use them it tells me I can't use them.
All I need now is an apple-scriptable Monitors Control Panel, so I can automatically change my resolution before I boot to linux, but leave it at 640x480 for MacOS ... Ian
