I'm guessing that extra wires wouldn't cost much if you arranged them now. Definitely wire the surrounds (at the sides, right ?) as top priority.
Note that on a 7.1 receiver you can use the last two channels for 7.1 or for remote speakers but not both, which I guess is why you're asking good questions now
I would have to vote for wiring 7.1 and picking up a cheap used receiver to drive speakers upstairs... unless you're also willing to wire the IR extender you need to let you control the downstairs receiver from upstairs. It's just too much of a pain running up and down stairs to change volume or switch stations/CDs IMO.
Wire for 7.1. Do think hard about where the sub is going to go... if you're pretty sure it can go up front then side surrounds and rears is all you need.
EDIT -- you're not actually going to use in-wall speakers for surrounds or rears are you ? If so, may we have a chance to talk you out of it ?
