A very realistic setup is a 7.1 setup. You can only drive 7 channesls plus one subwoofer at a time. It only contains seven (only?) discrete channels of amplification within the unit.
So, if you want to get more out of the receiver, you may want to add rear surrounds or front presence speakers to your setup. If all of your wiring is being done inside the walls and it is new construction you may want to go ahead and add wiring from front presence speakers AND rear surrounds at the same time, then you can try both and decide later what you prefer. I imagine in a couple of years, we will see 9 discrete channels of amplification from Yamaha in the 2500 and you can run 9 speakers plus a sub at the same time.
A separate amp is not necessary as long as you are running a pretty normal setup. Just decide how you want the room (5.1, 6.1, or 7.1) then wire accordingly, and hook speakers up. You should be happy with the power out of the box.
