well, I just installed Leopard on my 12" PowerBook G4 with 1GB RAM, 867mhz and a 160GB 5400 Hard Disk. Leopard is running pretty well in my opinion but it's definitely slower than my MacBook Pro. I'm using it on a daily business for surfing, reading RSS, mail, edit documents and reading online magazines. It has lived up to my expectations so far.
I would not really suggest Photoshop and stuff like that which demands a lot from the system. Well, that's my experience and opinion. Try it out and see if it works for you.