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