Leopard for me is great on low end G4s. The thing that makes the most difference is having a decent graphics chip.
For example, my G4 upgraded Pismo (thanks dosdude1) has a rage 128, which doesn't have Core Image, or Quartz Extreme. Leopard does not run very well on it. It can handle everything just as well as Tiger for most part but the sluggish UI makes it seem slower than it is.
I have a Mystic G4, dual 500Mhz (previous setup was identical in a sawtooth with a dual 450Mhz before I found the mystic) SATA card with probably a combined total of 2TB of storage, 2GB of ram, and it has a Geforce 6200 GPU. Leopard flies on it. It's got Tiger installed on one of the other drives but I seldom use it. The GF6200 is full QE/CI.
Similar story with my B&W. It's a G4 550Mhz, 1GB ram and a Geforce 5200. Leopard runs very well on it. The boot drive in it is running off an ATA133 card.
I also have a G4 Cube that runs Leopard. It's mostly maxed out as much as I can afford. 500Mhz, 1.5GB ram. It has a Radeon 7500 in it. The 7500 does not have Core Image, but it does have QE. I have had zero problems running Leopard on it and I find it quite smooth. When it had the factory Rage 128 you'd think it was a completely different computer.
I'm one of the few that stick Leopard on everything I can. Contrary to popular belief it is not slower than Tiger. It is a very modern OS even by todays standards. It uses a bit more memory, but thats really it. I've never boarded the "Leopard is slower than Tiger bandwagon" because frankly it's just false.
Playing YouTube on any PPC mac is chore, even on a G5. This is because youtube is utter garbage and our older operating systems have basically zero browser support. Youtube runs like crap on my Windows 7 based 2.6Ghz Pentium 4 IBM PC. It obviously has a modern browser, but youtube still sucks on it. If you're trying to play it in-browser, unless you've got a G5 I suggest you stop. The only reliable way I've found is to download the youtube video and play it in VLC or coreplayer. I'm able to play it in 1080 on my 1.5Ghz PowerBook G4 (smoothly) that way, and 720 on the Mystic G4 plays okay.