You'll get more bang from upgrading your Front 3 first... match them up and you're solid. Beyond that, don't skimp on a good quality Sub! After you upgrade your Front 3, get the best single sub you can, then save and buy a second one later... Or just buy them both at the same time.

I <3 my Outlaw X-13s!
But seriously, Shady's suggestions are solid. Also, if they are available near you, I really liked my experience with Monitor Audio Silver Series. But I ordered Philharmonic.

SVS is nice in that they match up between Prime and Ultra lines. So you could do Ulras up front and primes everywhere else.