Memory issues Mac OSX Lion.

I have the early 2011 MacBook Pro 2.3 ghz. I installed two 4gb rams cards (corsair) which had great reviews for the MacBook and it worked great for a few months. Today, even with 8gb of RAM onboard, I find the OS to be sluggish. I use iSTAT NANO to monitor my Mac and it tells me sometimes that I have only have 55mb free. Is it just me, or is this crazy! I don't use any software that is heavy on the cpu, mostly just Chrome and the Microsoft office suite along with iPhoto and iTunes. What should I do?