Having said that though, personally, I would use the PII as the firewall, backup, file server etc etc. The new fast machine as the family desktop and ditch the other two.
Using the same machine as a backup file server and a firewall seems a little foolhardy to me, especially given that there are spare machines to play with.
A fortnight ago my landlady went out into the garage and found that the breaker had gone several days before and all the food in her freezer had gone, including her prize winning rowanberries - boohoo! Made me jump up and start trying to figure out a method for an alarm light to come on in the house whenever garage power was off. Now that's fairly simple to do and doesn't need a computer, but there are a huge number of related things that you could do and you could sensibly put an ancient machine at the heart of that. The older the better even, because it will draw less power when left on for long periods of time. My pap used to have a set of lights programmed to change in a random way and he kept an ancient box running for many years doing little other than that. Light up, gradually dim, switch off, the whole lot. Classic anti-burglar mechanism jazzed up to make it plausible day after day.
Regards, Max