Still, with that much RAM available, the machine has not slowed down due to not enough RAM.
It could be time to start doing some regular maintenance.
Every so often, if using Safari - hit the Reset Safari option in the menu and let it clear all that old junk out - I only do this about once a month. There will be similar options for FF and Opera.
Also, grab Onyx or Main Menu - both free - and put them through their paces. Have used and like both - currently using Onyx - try the automation tab and check off everything in the Maintenance and Cleaning sections every couple of weeks or so.
Don't know how full your drive is getting, but do keep in mind, you will notice some slow down as you start hitting the 50-60% full range. Shouldn't be much, but this is normal with any drive and any OS.