Your Pioneer should have plenty of power for most speakers. I know it will have plenty of power for Kappa'* without needing any additional amps. If you have a sub in the trunk, then you really don't need to replace the 6x9'* in the back unless you want to get high frequencies in the rear seat as well. Some people remove the 6x9'* altogether when adding a sub.
You can spend $100-200 on Kappas(either components or not) for the front seat and get great mid'* and high'* to complement your Rockford.
I'm no expert on sub amps, but if you only have one 12" Rockford, I'd say get a brand name amp(Rockford, Infinity, JBL, Sony, Alpine) with an RMS rating similar to your Rockford'* RMS rating. You'll only need a single channel mono amp since it is one speaker. If I remember correctly, these are called "class D" amps. Don't go cheap on your sub amp or you won't get good sound out of it. Figure about $.50 to $1 per RMS watt for a good amp. DO NOT get the crap that those gypsies sell at convention centers around the country.