If your RAM usage rarely goes over 65%, you're not going to benefit from adding more RAM.
Switching to faster clock speed RAM typically only gets you 1%-2% better performance in real world use (i.e. not in benchmarks which stress RAM speed exclusively).
Save your money for a new system in a couple years. If you haven't yet, switch your HDD to a SSD. That is by far the biggest bang-for-the buck speed improvement you can make on a computer.