When you say a dual quad core power mac….I assume you mean an intel mac. Yes, it will be plenty of horsepower but I would recommend setting up a second and third HD as a RAID 0 for holding your footage. Motion comes in combo with Final Cut Pro in Final Cut Studio. The Primatte RT keyer is within Motion. Final Cut converts the AVCHD to an Apple Pro res format. From there, Final Cut exports to Motion for the keying. Save it in Motion and the keyed footage is magically back in Final Cut. You can output it any way you like at this point. So I no longer use Premiere Pro as a result. Certainly, if you wanted to edit the footage in PPro, you could export it from FCP and do that. PPro will eventually support AVCHD but I got tired of waiting.
As for purchasing Final Cut Studio, the only way to catch a significant break on the high cost is to purchase the educational version. This “educational” version is identical to the usual version but you cannot use it for commercial use.