8 Gigs would be sufficient for many users. I've used 8 in my imac 2009 all the way, but it struggled with larges multilayer photoshop files (2gb+ files). SOme times I had shut down a lot of smaller programs, like sparrow, twitter etc...because I had to give everything to photoshop. Now, with 32 gb I dont need to worry. i can open the largest images I would ever use in addition to not caring what apps are open. So, it's convenience in addition to smoothness and not needing to worry - that for a cost of about 100Euros for the 32gb. So its way worth it...it's so cheap that maxing it out is an obvious choice.
27" iMac i7 2012 | 13" MacBook Air - 2013 | iPhone 5s | iPad Air ...etc