I bought imac i5 27 2011 came with 4gig ram, had too many page outs, and the coloured ball kept coming on a lot, went with 16 gig and then 32 gig of ram. It is a definite change for the better, more programs open, and no page outs.
I'm with Perry on this one. Unless you are making the next Toy Story movie on that thing, it is not very likely you will ever use more than 8GB of RAM.
Just run it a few days and check for page outs.
Here is mine after several days running about every app you could imagine. (Not making Toy Story though )
Generally you know if you really need the extra RAM - all these people dropping 32GB into their new Macs right after opening the box are really just going for overkill at the moment.
Even 16GB is almost overkill. I have 16GB in mine (added an extra 8 on top of the stock RAM with another 2x4GB for considerably less than the 32GB crowd paid for their 4x8GB) and I've never had a page out despite very heavy use - I even threw 8GB at the virtual machine just for fun and still never hit a page out with both OSes running together.
At this point I'd say that 8GB is more than likely enough, 16GB by adding a pair of 4GB modules is extremely cheap and will be good for a few years for 95% of users, and 32 GB is just overkill
If in a year you are feeling the pinch (unlikely, but still), the 8GB sticks will be much cheaper than they are now.