Speaker Setup

Deltagunner00

Hey Guys!  New guy here. Cool site with lots of good info.
I am switching rooms in my basement and need recommendations on a speaker setup. I built the house in 2008 so I ran all wires behind drywall with speaker wire plates and mounts in certain locations. I have just switched rooms in the basement and only have my left, center and right front speakers setup with my sub. I didn't want to run all that wire along the floor to my back speakers so that's the way it sits currently.
My setup consists of a dated Magnavox MFX 450. My question is, can I buy some better rear surround speakers and have enough power from this receiver to run them?  Or, should I skip the rear speakers and upgrade my front speakers?  Is it best to upgrade the left and right or center?  Also, all the speakers are just the ones that came with the unit. The receiver does what I need it to and would like to just upgrade speakers if the receiver will handle it. Should I upgrade sub as well?  
95% of use is movies and sports with just a little music ran through the system.
Thanks for any help you can provide...excited to upgrade some stuff.