Sorry, didn't see you had SVS sub already. If you do decide to upgrade front 3 speakers you could use your rears as surrounds. haha But if you really want surrounds now you can get bipole/dipole if you put them on the sides of wall. Emp tek has a sale and fluance has bipole surrounds even cheaper at $120 pair.
Main theater: Sony 45es, 120 inch 16:9 screen, Panny BDT500, Rotel RMB-1075, Rotel RSP-1068, Klipsch RP-280F, RP-450C, RP-160M, SVS PB13-Ultra x 2, Axiom EP500v2, Monster Power Conditioner, GIK acoustic panels
TV Room: Panasonic 60" plasma, Integra DTR-30.3, Axiom Audio M60, VP150, QS8