I am about to update my 2007 iMac from Snow Leopard (10.6.8) to Yosemite. The iMac has 4GB RAM and I plan to go to 6GB based on recommendations around the web (since that's the max). For the past 6-9 months the iMac has been running real sluggish. Activity Monitor indicates memory leaks with Java, Fonts, and even Safari. I have no idea what caused this, and I've nuked about 2/3rds of the fonts I had installed. Prior to this the iMac ran quite smooth. My question is this: Should I rebuild the hard drive with 10.6.8 all nice and clean and THEN upgrade to Yosemite? I'd like to rid myself of years and years of 'baggage' dating back to computer migrations from the late 20th century (literally) and start fresh. My natural concern is having to (a) then manually retrieve a lot of files that I need as I go and (b) the inconvenience of having Safari/Keychain relearn passwords. Does it make more sense to go that route, or should I just soldier ahead as-is? I have an external drive, so I'll clone this drive first before doing anything.