Heck yeah, the original minis were PPC powered- it's why they were so cheap. Unfortunately, the logic board went bad on mine, so it's just a gutted box at this point.
But yes, as others have said, you'll need to get a copy of 10.5, which is the latest you'll be able to install on that machine. Fortunately, I found the transition from 10.4 to 10.5 to be one of the best upgrades Apple's ever offered. Realistically, you're probably not going to get a copy of Leopard for very cheap, but it can be had for around $50 occasionally. I'd recommend you post a want-ad to the LEM swap.