well if you are short ram your computer swaps to your hdd which slows you up. In your case you would swap to an ssd which is less of a problem, just go to your activity monitor. on mine I have 1.38gb page
in with 60.8 mb page out and 61.1mb swaps. this is with 8gb ram on a 2011 mini and a large ssd as the booter. So 4 % of the time I use the slower ssd since I exceeded my 8gb ram. 4% is 60 million over 1.4 billion. or close enough.
I do not notice the slower ssd when I swap . yeah it is more writes on my ssd but not too many. look at your activity monitor and if you do 1 billion in and 400 million out or 40% you are using your ssd more. your ssd is harder to change and cost more then ram. so why not drop in the ram. but certainly at least read the activity monitor on and off for a week. it resets with each boot. so don't boot until you check the ram use.
In a week you will be more informed. by the way you can buy 1 8gb stick and then have 10gb ram. 40 bucks or so.
http://www.jr.com/crucial/pe/CRT_102464BF160/
these run 38 to 45