The graphics corruption could be just a graphics card, or problems with the logic board, or could be (less likely) just a problem with RAM (or a combination of those) If you really want to resurrect this old iMac, you could start with replacing the RAM, check for any change in booting, then try replacing the battery (a button battery on the front of the logic board. After that, it gets quite time-consuming (really time wasting at this point). Parts (actual working parts) are getting more difficult to find. Easiest way is to find another like Mac model (working or not), and start swapping parts to see if you get anything working. Done this a few times (it's a hobby), and have a 20-inch, early 2008, fully working, running Sonoma/OCLP, getting ready to update to Sequoia when that is released next week. Some will say working on a Mac that is approaching 20 years old is never worth doing. I choose to disagree.