I have a quicksilver g4, and leopard really feels slow on it. Even being a 933mhz with 1.5gb of RAM. I think that the "feeling" is mostly related to the video card, which is a Geforce 4 Ti 4600. Does not enable "core image"...
Tiger also feel a little sluggish. Actually I never ever had a quick feeling on any OSX on any HW. My wife has a Core Duo Macbook Pro and I still think it sucks!
I'm a huge Windows fan. (Don't kill me guys!)