You can get a 7.1 receiver and use it with 1 speaker! I've got a 7.1 receiver, and went from 2.1 to 4.1 to 5.1. I will probably not be going to 7.1 at any time in the near future. As for THX, you certainly don't need that, unless you like little shiny nameplates.
You should make sure you account for a good sub in that $3500. Let's see:
$500-700 for a reciever. You can get decent deals on refurbs or last year's models.
$600-1000 for the sub, depending on which way you go. Consider Axiom, SVS, and Hsu.
$100-200 for cables, unless you've got some. You don't need boutique cables, just good solid ones. Have a look at Blue Jeans Cable, Parts Express, or Axiom
That leaves plenty for speakers, unless I've forgotten something.
I'll let someone weigh in on which package to get (I'm not good at judging room size), but you should be able to get a nice set for that.
