If the mini mac is a G4, (unless you have the brand new Intel version), then the latest 'live' version of power pc Ubuntu would be the choice. With OS X there is no way I know of to boot from a firewire hard drive unless it's got OS X on it.
If you use the install version of Ubuntu, then you'll have to partition you mini's hard drive, and probably hose both OS's eventually. I've never been a fan of dual-boot.
But - if you cloned your internal hd off to a firewire drive and booted from it a few times to make sure it works, then you could install Ubuntu on the internal and boot from the fw drive when you want to run OS X.
I have Virtual PC. It will run Dynebolic live pc linux. Ubuntu's pc version won't run in VPC. Knoppix and Free BeOS won't run either. Knoppix has video problems at run time and BeOS can't find the virtual hd, but the live version runs okay. If I 'formatted' a virtual hd first, BeOs might run, but I didn't think it was worth the time.
If you use the OS X version of any type of Linux, then if you want to go on the web with it, you must have dsl or cable through your ethernet adapter or 'maybe' wireless. There is no proper way to put a hardware modem on a G4. My Hayes Accura works through a pda usb to serial adapter in Panther only because the adapter has OS X drivers.