Originally Posted by mijotter
So what IS the difference in a 7.1 and 5.1 receiver if not how man speakers it can handle properly?
That IS the main difference. But since 5.1 receivers are typically at the lower end of a company's product line and 7.1/9.1 receivers are at the top end of the product line, the latter means that you get things like: better room correction, additional surround processing choices, more connectivity, greater amplifier power, etc. So it's worth getting those features, even if you never plan on going beyond a 5.1-speaker layout.
I'll also echo what BllDo posted: since you're in the construction phase, this is the best time to run wire for additional speakers. To that end, a couple of quick questions about your set-up. What are your room dimensions (LxW)? And where is your main listening position going to be (i.e., how far from the back wall)?