Best rear center channel speaker for my new setup?

[thirdkind]

I'm setting up my first home theater, and I need to get a rear center channel speaker for DTS-ES. Here's what I've purchased so far:

Onkyo TX-DS797 receiver
Toshiba SD9200 DVD player
Polk Audio speakers:
- RT800i front left/right
- CS400i front center
- f/x 500i rear left/right
- PSW350 sub

I thought about getting another CS400i for the rear center, but I think that might be overkill. The next step down is the CS245i, but it's not voice matched to the rest of the speakers.

Is the CS400i too much for the rear center? If so, what's the best model to get?

Thanks :)