The iMac, while a fine machine is basically a closed you get to use what you pay for machine.
Many people come by looking to replace the HD/optical, boost memory, and/or upgrade video/CPU.
While you can replace the drives and memory by cracking open the box, if you miss any thermal pads/past locations the machine could suffer thermal shutdowns.
Outgrow the video and you'll live with it.
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The G5 1.6 is a more open machine, and you can play with the drives, memory, and video cards all you want.
But if you're into video/graphics -- you have only 1 memory upgrade in the machine. After that it become a choice of which sticks to toss to upgrade the next time. (2 dimm pairs)
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For a video/graphics person -- it's the G5 1.8
The boost in speed ain't the key, it's the 8 memory slots. And the PCI-X that may be of use.
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Remember, with a new ATA cable -- you can drop an ATA-100/133 drive to the Optical chain of the G5s.
May not be the best location an will block some airflow. But it's a way to boot the machine when the two SATA drives are turned into a RAID.
Or a way to keep and old drive useful as an emergency boot volume that you keep on the shelf.