I agree with Michael. With speakers as efficient as those Klipsch, and the 805's very powerful amplifier section, I doubt you'll be using even 1/3 of the Onkyo's power to reach reference levels. I doubt that a separate amplifier would make a speck of difference in that system, unless you believe in the 'sound' of amplifiers driven below clipping. My speakers are 3dB less efficient than your Klipsch, taking twice as much power for the same apparent loudness, and the 805's amplifiers are still significant overkill. I think you'd be much better served by upgrading your surround speakers - Klipsch seems to have lots of options, and possibly your subwoofer. If your subwoofer has enough output for you, then instead I'd focus on room treatments. Putting the Onkyo's full output into any of your Klipsch speakers will result in 117dB, which is way above what your subwoofer will be able to generate. And that's rated output - the Onkyo will do more before clipping.
As I final note, I just saw that you were looking at the UPA-7. If you did get a separate amplifier, that one is not an upgrade at all from the 805! It's actually rated lower than the 805's built-in amplifiers. If you're going to go to a separate amplifier, my opinion is that it should have at least 2-3dB more headroom than what you're upgrading from - that's twice the power. Otherwise, you may not even notice the difference.
