I can attest to the fact that GarageBand does not do very well on a 400 MHz G3. It will play some instruments for a while but usually overloads and has to stop. It doesn’t crash or anything – just stops. This isn’t a problem though because it does work some and requires a 600 MHz G3.
iPhoto on the other hand is a much better story. It is noticably improved on my slow computer. I can very easily scroll through over 1,000 images and resize them on the fly.