The one speaker in the middle is probably your center channel. Why not go all on out and just get dolby digital 5.1 surround. Req below
4 speakers (one sub)
-Main bookself speakers up front (2 obviously)-I just used my two bookself from my stereo setup a while back.
-center channel (you already have) in center. I have a JBL which was nearly 200 alone, but you dont have to go that deep
-2 rear speakers in the back (dont need to be expensive I just bought some off ebay for like 15-30 bucks) It was a good price. To hook the rear up in the back just get some 8 gauge speaker wire, just tuck it under your trim in your room-good solution and get those white plastic wire covers when you go up the walls with wires.
-And last a sub-getting cheaper and cheaper (probably get an average one for use with games for 30-50)
-of course you will need a dolby digital decoder on your reciever just make sure of that. Also make sure its got atleast one digital coaxial input and or optical input. I went with optical because on my 360 a/v cables I have a optical slot. If you have xbox monster cables you probably have one to.
(my reciever is a yamaha htr5830 it was around 170 at bestbuy)
I know its a lot to think about but you will really enjoy the sound. Just something to think about.
please note this is for an average but pretty impressive setup, mainly for a small room.