I have an Onkyo HT-S7800 and i would like to replace my existing center speaker with Klipsch RP-250C. Is it compatible with my existing system? Is Klipsch RP-250C worth the upgrade from my existing center speaker?
I would consider replacing the 3 front speakers with Klipsch just so you're timbre matched but who knows, maybe you'll get lucky with the Onkyo Left and Right matching a Klipsch center channel. As far as the Onkyo handling it, shouldn't be an issue.
You'll find many in agreement on Klipsch equipment of any config wherever you go and whoever you ask. I am not going to say its a bad choice. But because you are able to blend any speaker with other existing speakers in your current system i suggest you start thinking about a goal of bigger and better speakers all around in your upgrade.
The Elac speakers i've noticed have had some very fine reviews and the Elac C6.2 (center channel) looks like another fine product. I prefer the Elac's sweet sounding silk dome tweeter above Klipsch's dry tinny horn loaded tweeter. Then the Elac's 6.5" mid drivers are very capable drivers much as the Klipsch 5.25" driver but again i prefer the 6.5" aramid driver over the Klipsch metalic 5.25" the added bonus are the forward firing ports of the Elac.
Elac is relatively new to the market and their prices are still low.
Of course i would rather you buy Dynaudio, or Totem or even MonitorAudio..., actually Monitor is another near your price range.