According to several sites, my mini can boot from an external firewire hard drive, no problem. Using SuperDuper, it was easy for me to create an image of my boot drive on my 80GB, 7200RPM firewire/USB 2.0 drive. However, that just got me a faster... not bigger boot drive.
I also have a 200GB, 5400RPM firewire/USB 2.0 drive. That would give me the space increase, but not the performance boost, especially over firewire. Plus, it's currently my backup drive for the various computers we have around the house (and then some.)
I have tried every combination... one FW, one USB, both daisy chained through FW, one in front of the other... and the mini only seems to like one combination: the internal drive and one external drive. It sees both drives and can do things to both drives... but it sometimes hangs in the middle of operations. I don't know if that's a limitation of the mini, or the interface standards, but I can't seem to get around it.
Ah well, I guess I'll just have to save up for one of these.