I just "replaced" my 2010 i7 2.8 GHz iMac with 12 GB RAM with the new 2012 model. On the 2010 iMac I originally had Snow Leopard, then skipped Lion and then upgraded to Mountain Lion which I love. Lately I'm seeing more beach balls on the "old iMac" as programs load and do work and I can't help but think that if I were to erase and reformat the 1TB hard drive and reinstall a clean version of ML and reinstall all my apps, etc. the computer would run like new again. Would doing a clean, fresh reinstall really make that much of a difference in speed? Or am I just noticing the difference because of the lightening fast speed of the new iMac? It's a lot of work to reconfigure everything and don't want to do it unless there would be a noticeable speed improvement.