To be honest Roger, I haven't toyed with Kerky myself, so one of the KT regulars would be best qualified to answer how long renders would take, but I gather it depends on method used as well as scene complexity. I don't know if the Kerkythea forum is back up yet, maybe have a look there. Kerkythea can make use of as many CPU cores as you can throw at it - groovy huh?!
To my knowledge, there are no 8-core processors commercially available. However there are machines equipped with motherboards that accept two 4-core CPU's delivering a 8-core machine. Be warned, they are freakishly expensive, due to the CPU's used as well as the RAM requirements (typically uses ECC RAM) and powersupply needed together with cooling for that monster.
I just built myself a new 4-core machine with 4GB of RAM a week or so ago and as soon as I've got a little spare time I'm starting rendering.