Samsung S3 - 40mb free ram of 1.5 gigs?
on the Play Store there are several utilities to clean Phone and speed up processes. My favourite is this:
https://play.google.com/store/apps/details?id=com.cleanmaster.mguard

Your excessive use of system RAM is (maybe) due to some persistently active process on your phone. Follow this guide to see yours (and maybe evaulate a uninstallation of some):
http://www.wikihow.com/See-What-Apps-Are-Currently-Running-on-Android

By the way, can you please tell us more information about your Android version?

Best regards,

Luca