If you're in X and can't exit any more gracefully, do Ctrl-Alt-Backspace. If you're at a log-in screen, it ought to give you the option to switch. There's a way to pass the option at boot, I believe, but I can't think of it. Try a boot/rescue disk, maybe, if nothing else works. Go in and edit inittab, I think.
These default GUI login screens and runlevels in X are ridiculous to me. The default should be 3 and you should switch when everything's probably configured - and I wouldn't even then. But distros have to be "pretty" now.
I wonder if you didn't blow your monitor with faulty settings if you say it was working and is now screwed up.
Hope that's not the case. Switching cards without changing the configuration again is, uh, not likely to work well, to say the least.
I don't know anything about serial mice but you are going to need special configuration. Onboard video really shouldn't be a problem unless it's ancient or brand new, I guess. I assume Mandrake has some whizbang X-configuration tool, so try that again, or do 'xf86config'.