Make a note of the ' Model Identifier' entry. Upgrading RAM in an i Mac is a simple task, even for novice Mac users.Apple placed either two or four memory slots in the bottom of each i Mac.
These early Intel-based i Macs are probably not worth the time and cost to update.
This firmware update and related text is from Apple's
Another option is to forgo upgrading the internal hard drive, and instead, add an external model.
You can use an external hard drive that you connect to your i Mac, by USB, Fire Wire, or Thunderbolt, as your startup drive or as extra storage space.
The key to performing an i Mac memory upgrade is selecting the proper RAM type.
Check the i Mac Models list, below, for the RAM type for your model, as well as the maximum amount of RAM that can be installed.
artnum=75130 Due to difficulty downloading from Apple's site I have made it available here.
This is the 4.1.9 i Mac firmware for slot-load (350 - 700 Mhz) i Macs.
It's possible to upgrade the hard drive yourself, but I generally don't recommend it except for experienced Mac DIYers who are comfortable taking apart something that wasn't designed to be easily taken apart.
For an example of the difficulty involved, check out this two-part video from Small Dog Electronics on replacing the hard drive in an early 2006 i Mac: Remember, these two videos are only for the first-generation Intel i Mac.