You cannot replace a conventional “Hard drive” in this model iMac.

DO NOT attempt this upgrade until you understand two important facts:

1) The drive in your iMac MUST BE a ‘SSD’ or a ‘Fusion drive’. If it is a conventional HDD, stop. There is NO PCIe CONNECTOR inside to connect a new blade drive.

2) Click on the link in this iFixit for "clone your existing hard drive". All the more recent Apple CPU's are now classified as 'T2 secure chips'. Cloning is no longer straightforward, e.g. "Hold down Opt key during restart, etc." You have to bypass/disable certain security features on your iMac, and this is somewhat of a lengthy process. If you do not, when you try to boot from an external drive you will see nothing but a black screen.

Excellent step by step guide, thank you. I will definitely refer to this when I upgrade my 250G SSD which, BTW, is WAY too small. There's only 100G left, and only apps are Logic Pro X, Numbers/Keynote/Pages, iTunes & a few utilities... hardly any actual work

If your G5 is the liquid cooled PPC type these instructions are on the money. The power supply needs to be tipped/angled in about 45* before it will clear the case and pull free. There’s not enough room in the case to do this easily. It scrapes hard on the case and it takes some force. Mind the posts that align the processor to the motherboard. Do not bend them when removing the power module. Move the thick bundled wires to make as much free room as you can before you muscle it. It really does have to be forced out. My G5 was cutting out in the middle of use, then coming back on a few seconds later. I believe the cause was extremely dirty power supply fans. I’m glad I took it apart so I could really clean everything, but in a pinch try blowing compressed air into just the bottom, front & back, while using a vacuum at the other end. That would probably have done it.