Download tenfourfox if you are on 10.4, Aurorafox if you are 10.5, enable plugins and get the latest hacked "11.2" flash, if you absolutely must. Be aware that flash 10.1 is very unsecure (emphasis on the very) on PPC mac now, you are much better off avoiding it if you can.
Mactubes and or Youview will solve the youtube dilema for you. In 10.5 Leopard webkit with Click to plugin extension will also solve it. I am surprised at how much webvideo Click to Plugin can "see" and therefore play in quicktime. It will give you far, far, far superior playback to flash on your PPC macs, unless they are very fast G4's and G5's.
There are also Omniweb, Stainless, Roccat, and processor optimized Camino builds still out there and relatively current for PPC macs. And don't forget iCab. Its my browser of choice these days on PPC OS X, and I predict will be the last one standing two to three years from now.