Hi. I am using an old iMac for the next month while i wait to buy a new mbp [when they are eventually released]. The iMac is old, it is a 2.4 Intel Core 2 Duo with 4GB of Ram, hd 250GB. I borrowed it off a friend & he said I can reformat it, etc so that I can get it to run as fast as possible for using Photoshop CS5 & Capture One, & very occasionally Final Cut. So, what I want to know is: which Mac OS will make it run fastest? Am I best to put an older version of Mac OSX on it? If I put the latest OS on, will it make it slower & clunky or will it run smoother? Can anyone with experience help? thanks.