I've been using FCP 2 (FCP 5.1.4) on an PowerMac G5 (1.8 KHz, 4 GB RAM) with Tiger (10.5.4) without problems. I think you can go up to FCP 6 with that setup. Of course, it's outdated, but still workable.
Apple hasn't come up with a really, really good up-to-date MacPro to make me leave my old PPC.
Alberto