It's hard to justify the purchase of a PowerPC Mac, unless you're short on money and really want to kinda get familiar with OS X. You can load your flavor of choice PowerPC Linux also and turn it into a server or something, but unfortunately Apple is killing OS X support for the PowerPC series very quickly. Aperature 3 is now Intel only and even Garage Band was limited in some features upon it's release a year ago. Leopard updates are only critical security releases at this point.
As for Leopard running slow on a PowerMac G5, I have to disagree. I've had it running on a dual 1.8 and dual 2.0 without any noticeable slowness issues. I will say that the single processor units were slow as hell in general though, our advertising department just got rid of theirs and I'm very thankful that I no longer have to support them.
Watch out for the faster 2.5 and 2.7GHz units, they are water cooled and there's a long history of then leaking and failing.
