First of all, you will be one happy person with that system you describe. As to whether to upgrade to QS8's, I'd suggest calling Axiom, as they will be honest about whether the 8's will give you much more than the 4's in your size room. Having said that, if you will end up regretting not getting the 8's in the first place, then I'd say just spring for them now. They're amazing.
And great choice with the SVS. I use the PB12-ISD, and it is just awesome. The ones you list should be great choices.
As for receivers, the 2805 would have more than enough juice to power your system. I would also recommend you take a look at the Harmon Kardon 435 or 635. I just upgraded my older H/K receiver to the 635, and it is one fine receiver. The 435 and 635 have a great setup system, that can EQ your room if you want. The 435 may be more in your price range, and has most of what the 635 has. And the H/K sound just works so well with Axioms.
Best of luck.
