if you have 4GB right now, i bet you probably notice a lot of beach balls and hang time. but if you put 16GB in there, everything will be soo much smoother and snappier, and very little to no beach balls or hang time at all, even if you don't restart your mac for a month!
just make sure to run memtest86 on the ram sticks when you get them before you commit to keeping them. run it for about 2 hours and if you get errors, package them up and send them back for another set. chances are you wouldn't get errors, but play it safe to save all the frustration down the road.