Rate these on order of importance:

1,5,4,3,2

1, 5 , 2, 4, 3.

Marco-
The Pizza analogy was great. Do you want a Smoking Loon Cab or Merlot with that pie? ;)

1 and 5 should really be tied for first--it's the interaction between the two that makes the sound. Everything else pales by comparison.

In my experience room placement is critical to getting the most out of your system. So I would start there. Once you've found a placement withint the room that works it can only get better with changing the other items on your list, though tweaking will be necessary with things like specific speaker plcaement once you change those. I've heard very modest systems with good placement kick the wazoo out of great systems with poor placement.

I agree that speaker choice is next, though closely tied in with placement of course.