Would a center speaker matter?

[basite]

it might, but I'd recommend getting rear speakers first, and using the ghost mode on your receiver...

for a 5.1 system however, you're going to have to get a 5.1 receiver. What you have now is a stereo receiver, with DSP effects, so you can't get a real surround sound now. You get an upmixed stereo sound.

so I'd actually get a new receiver first...

Keep them spinning,
Bert.

[Hal]

Thanks. I'll save for the receiver.

[BadAssJazz]

Thanks. I'll save for the receiver.

The good news is that you can get a home theater receiver at incredible prices these days. You don't have to go "top of the line", since solid entry level receivers that work well can be had for $400 or less. In fact, if I were just starting out now, and on a tight budget, I'd have no qualms about picking up either one of these units:

Yamaha RX-V363 ~ $200 online (75 wpc X 5)
Yamaha HTR-6030 ~ $200 online (100 wpc X 5)
Pioneer VSX-516 ~ $200 online (100 wpc X 7)
Onkyo TX-SR506 ~ $300 online (75 wpc X 7)
Yamaha RX-V463 ~ $300 online (105 wpc X 5)
Denon AVR-1508 ~ $350 online (75 wpc X 7)


You'll need to research these receivers to see if their features suit your needs, but it's a good starting point. Obviously, if you have a larger budget, then skies the limit.

Now for the bad news. Yes, you do need that center speaker. Yes, it should match your mains. Yes, if you skimp or cut corners on your choice of speakers, it will only lead to buyer's remorse and the inevitable upgrade...much sooner, than later. So take your time to audition speakers (if possible), give yourself a realistic budget, and exercise as much patience as possible putting together a system that sounds good to your ear.

And when you're all done, come back here and tell us what you bought.