Tony,
For live performance, I use (with the KN5000) and highy recommend the Digitech Vocalist Workstation EX. (approx $425 US dollars).
http://www.digitech.com/products/productpgs/vws.htm The digitech harmonizer sound quality and features are far superior to the pathetic sounding one included in the KN6500/6000. The Digitech unit will sit conveniently on top of my KN5000 keyboard (though it does cover up the right speaker) for quick access during live performance. I really like the convenient sliders which make quick lead/harmony balance changes easy.
Another option (abeit more expensive) is the Voice Prism by TC Helicon. I checked it out at the NAMM show and it sounds even better than the Digitech and offers even more fantastic featues.
http://www.tc-helicon.tc I think the Voice Prism is perfectly suited for studio work, not live performance though, as it's a rack mount type unit (so can't easily sit on top of the KN keyboard (for easy access) and I would find the tiny knobs hard to access while performing live. Still, it's a terrific unit which I won't mind owning myself.
- Scott
http://scottyee.com