Matt, there is a pretty big different between a receiver and amp. When referring to an "amplifier" most of us will assume you're talking about a separate amp. Yes, receivers have amps built into them. But, they are usually referred to as a receiver. These words are not usually interchangeable as this will cause confusion.
for my two cents. I currently have 3 separate amps for my room, which is 18x17ft. Preciously I was using a 5 channel marantz amp which is rated for 135w per channel, an mm9000. But, that amp was being driven into the clipping region; so, I bought larger amps.
more power buys you just that, more power; like John said, which I agree with. More power does not = better sound, it just means you can listen at a higher volume without driving the amp into the clipping region which causes distortion and destroys drivers.
hope this helps.
