How does one change X windows resolution at boot time from a usb drive?

[Robert Key <rob54321@mighty.co.za>]

 HI I boot ubuntu from my usb drive having installed it with

usb-creator-gtk. The problem is X is configured to use a mode that my 
nvidia card does not support. Is there a boot parameter I can use to 
change X's mode?
Thanks,
 Rob Key