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.