I don't think Brown's value decreases becuase of McNair's injuries. Volek is a decent QB, and he actually gets the ball to Derek Mason better than McNair seems to.
I think Brown's value does decrease because of his recurring injuries. He's been down for a few weeks now. Will he be able to pick it back up?
I think this week v. Bengals will be a good indicator. They're last in run defense. If he can't get it done against them, he jsut can't get it done. If he doesn't, I think it will be hard to trade him for good value, because every keen owner will have clued into this.
If you want to trade Brown, best to do it now. If he fails this Sunday, his value goes in the pooper and you'll prolly have to hang onto him.
As for fair value: at WR - I'd consider Horn, Andre Johnson, maybe even buy low on Harrison or Holt if you find a still itchy owner, Roy Williams, CJ (values probably high right now), Javon Walker.
at QB - maybe Leftwich, Carr, Favre.