Current speaker setup.
Since you connect a stereo source (TV) and have sound from all speakers, you must have an amplifier some place (in the wall, closet, etc.) which takes the line level input and connects 3 speakers to the Left channel and 3 speakers to the right channel. This amp might all ready have a volume control.
This is not surround sound. It is stereo audio comming out of 3 pairs of speakers. Verify for youself that all speakers are playing the same thing.
If you are happy with the current setup, then leave it as is, wire in a volume control, and connect all audio through the TV.
If you want 5.1 or 7.1 surround sound, you will need a receiver with speaker wires from each speaker to the back of the receiver. With this configuration, you can play a 5.1 Dolby DVD with special effects that pan around the room, etc.
As others have suggested, you should consider a speaker upgrade if you are buying a receiver and rewiring. Check out the bookshelf / sub systems from SVS and AV123.