What Macs can you upgrade?
he G3 and G4 iMacs are a pain to get into (the G4s especially, since you need to reapply thermal paste), but the G5 range and the early Intel models are much easier. If all you want to do is upgrade the RAM, in most iMac models you can remove a plate at the bottom, pop out the RAM it came with then firmly push in your new chips.
Replacing anything else, notably the hard disk, can be more involved, but for those G5 and early Intel iMacs, it’s just a question of first detaching the screen or back panel. The glass panels of the
Taluminium iMacs up to and including the 2011 models are just held on by magnets, so you can attach suction cups then gently pull them away. Sadly, from the 2012 models, Apple started sticking its glass to the aluminium backs with adhesive. Not only is this trickier to open, but you need to apply fresh adhesive strips each time you open it and then want to close it up. Irritatingly, you’ll need to go to these lengths even to replace the RAM in post-2011 20.5-inch models.
For iMacs that have optical drives, consider replacing it with a second hard disk or SSD.