I've been running Firefox on my iMac, yet it seems exactly the same (if not slower) than my G5 with TenFourFox 45. 😕Web browsing on the intel Macs is a thousand times better than on PPCs, even when a G5 is running TFF. The latest versions of Chrome and Firefox will last a long time on ML, so you don't have to worry. I would put the iMac in a cabinet and connect the ACD from there, just my thoughts
Mountain Lion is still downloading, I'll have it installed soon.
That does sound like fun! 🙂 I'm sure it would run El Capitan just fine after all of that. I'll take a look at the prices if everything I need to upgrade. I don't want to spend too much on this if I have a perfectly working iMac and a G5 already. I'll probably end up doing everything but the processor, but, again, I haven't seen the prices yet, so I'm not sure.May want to upgrade the RAM if you choose on upgrading the os. Me, having another monitor sitting in front of my all-in-one would drive me nuts.
You could always get a replacement panel off the interweb and take the opportunity to upgrade the drive with an inexpensive Intel sata2 ssd. Upgrade the ram and the processor. Maybe it's just me; but that sounds like fun. While you are doing all that you would have the trusty old ppc to use