Apple - iLife
Many of the editing tools in iPhoto 5 are not available on a G3. Rumor has it displaying images on a G3 is very slow. This is a time honored way to force upgrades, one that Apple has practiced for many years.
I suspect there are similar stories for iDVD and iMovie HD, but I don't follow them closely. Bottom line -- don't go for iLife 5 if you're on a G3. (I still have iMovie 2 on my iBook!)