With 99% of receivers I suggest you use the amp in 5x mode with the 3 fronts and 2 surrounds and let the receiver amplify the two surround back speakers. Although if you have a Lexicon RV8, Denon 5805 or Yamaha Z9 or something similar, then it might be better to use the amp just for the two fronts in 2x mode.
I'll most likely end up with a Yamaha rx-v1500 or the 2400 receiver. Given that this will be used for 100% home theater, would I really be accomplishing anything by adding an amp?
Hard to say. Depends on your speakers, subwoofer, room size, listening levels and so on. Usually a good amp brings a clear improvement to most systems.
The room is less than 4500 cubic feet. I'll have Axioms all around (either M60 or M80 mains). I haven't shopped for subs yet but it'll be something in $4-500 range. Listening levels? Just a family of 4 watching movies...none of us are hearing impaired. I know it wouldn't hurt, just wondering if it's worth the extra few hundred for an amp.