This thread is pretty intriguing, but I feel those who mention "Use what works for you to be most productive" have the right idea.
I used to have my upgraded 1.25ghz iMac G4 sitting next to my mac pro:
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For a while it worked great for what I had wanted- which was a 2nd computer to the side that I could use when my mac pro was busy rendering or whatever. Something I could browse the web, chat, and watch videos on. It could barely do youtube with Mactubes, but it was watchable.
Time passed on, some of the websites I wanted to go on changed and got very slow to browse, and I started to notice the true age of the machine doing basic web browsing.
For around $250 I streamlined my iMac setup next to my mac pro. I upgraded to a early 2009 20" 2.66ghz Core2Duo model with 1066mhz ddr3 ram and the decent Nvidia 9400 GPU. Even got a bluetooth keyboard and bluetooth trackpad to reduce any unnecessary wires.
This newer iMac runs crazy circles around my old g4 iMac. So much so that I have been using it as my primary computer for everyday tasks to save a bit on the electricity bill. I think my 2012 mac pro & dual display setup uses 3 to 4 times the power of this iMac. Apple rates this iMac to use 60.7 watts of electricity at idle! Pretty low power.
Sure, it doesn't have the same beauty and charm of the old G4 I have, but its still a very pretty computer that more then serves what I originally wanted my G4 to do. Its also totally secure and able to run Yosemite if I wanted (Currently has Mavericks on it)
My PPC machines dont get much use any more, beyond my iBook 466. I have fun pulling that out and playing on it because its easy and small. My iMac G4 sits packed away carefully in a tote surrounded by foam and wrapped in bubble wrap.
You have to use what works best for you. For now my old iMac will stay in storage, I cant part with her as ive stuck way more into it then its worth (400gb 7200rpm hdd, 2gb ram, new ide dvd burner...) Plus I love the design too much.
I recently dug out my G4 iMac and wiped it from Leopard, installed Tiger on it instead. That alone brought a VERY noticeable speed boost to the old girl. Sure, it can run leopard, but leopard really slows it down a lot.