OP, I think you're fixating too much on a model/performance viewpoint.
As always, the faster the processor and the HD and the more RAM you have, the better the performance. But that's true on any PowerPC Mac.
To give you some perspective, I have run Leopard on a G4/450 PCI with 1.75GB ram and the Adobe CS4 suite. It's keep it's own in a production environment. Right now it's performing as a print server for the work MacPro with Leopard and about 192mb of RAM.
I've run Leopard on my personal 400mhz TiBook with 1GB ram. My son runs it on his 1Ghz Titanium DVI with 1GB ram. His Mac is a rocket. My daughter is running it on a 1.42 Ghz iBook G4, my wife on a 1Ghz 12" PowerBook G4 and I have a 1.2Ghz iBook G4 that it's running on too. All those machines with max ram.
Add ram, get the Mac you want. Leopard does best with at least 1GB ram. You can disable the eye candy and make some Leopard specific tweaks and it'll be fine.
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NOT TRUE!
I've had it run smooth on every Mac I've every installed it on.