I would say that the amount of RAM really depends on the apps you use. Tiger was not very heavy and the OS by itself works well with 256MB. It starts to be slow when you open apps that require a lot of RAM.

Personally, I have worked with a Powerbook G4 having 1GB of RAM for many years and never noticed any slowdown. Especially on tiger. I have done a lot of intense video editing and rendering on that machine and only replaced it in 2009 when it was time for me to move to HD videos.