Press F6, then ESC, then tag "-xforcevesa -vga=xxx" (xxx being the appropriate resolution code) onto the end of the parameters. The xforcevesa option might help, just because it forces a specific video driver. You could see if tagging "-xsetup" works, but I don't see it listed in the list of Ubuntu boot options.
Otherwise, your video card swap plan would work, except that, instead of installing a driver, just alter the config file (xorg.conf? I don't remember right now) and force X to use the lower resolution. I don't remember how to do this, but I know I needed to when swtiching from a big CRT to a lower-res LCD when I moved an Ubuntu 8.04 machine.