Never heard that one before. The only thing I've upgraded on my iMac g3 was the RAM (64 --> 576). Thing's gone for 7 years (well, six and a half... keyboard mostly died a few months ago), and I find it easier to have a really good system that simply depreciates slowly better than upgrading it.
Yeah that's the G3. We're past the G5 and are at Core 2 Duos. The only thing easily you can upgrade in the iMac is the ram. You can upgrade the HD, but that's a little harder. An upgradeable system would allow for GPU updates.