I have an aging imac running 10.6.8. (Purchased 2008) I know I should purchase a new one, but I do occasionally use the CD feature for some old graphics access, and really don’t want a third large screen. I generally kick and scream with every OS update that makes me also update (relearn) all my software programs (and not fond of renting Microsoft office!) So I’m thinking I would like to upgrade my RAM on the imac and purchase a mac book to ease myself into the transition to the new OS. THEN later upgrade to the new OS on the old computer. I love my big screen. Is my imac defunct or will it upgrade?
Unfortunately your iMac will no longer run the current OS (Sierra, 10.12). It will run the previous version (El Capitan, 10.11). You could in theory add more RAM and then upgrade to El Capitan, but I would also recommend replacing the hard drive with a solid state drive if you want to keep that older machine running for a few more years. This requires opening up the machine and is something that an experienced Apple tech should perform. However, since it will not run the latest OS anymore, it is questionable whether you would want to invest that type of money into this older machine.
As far as purchasing a new machine goes, if you really love a big screen, you could simply purchase a new iMac OR you could purchase a laptop and pair it with a larger monitor for when you are sitting at your desk (plus an external keyboard and mouse). That way you could have a mobile machine when you are away from your desk and a desktop-like experience at your desk.