Tiger will work fine on all G5s, G4s, and most G3s. With xpostfacto, even earlier Macs will work. But officially, most of the ones that work with Panther will work with Tiger. Basically, if you have firewire, it will work. I've even seen basic Core Image features work on a 32MB Radeon 9200.
Not to jump on you too much, but why do so many people think Tiger won't work on their fairly new systems? Do people really think Apple's going to release something no one can use? I mean, again no offense, but they're still selling that machine right now. And other Macs that are considerably slower.