About the only hardware spike from Halo Reach will be due to any special edition Halo bundles being offered. It aims squarely at the current Halo fanbase and is unlikely to pick up any significant number of new FPS fans.
As for Natal, I'd have to say that consumer reception depends almost entirely upon asking price for bundles as well as the co-release of any Natal based killer-app(s).
The only circumstance in which I see the 360 topping the sales charts for the month of September is if Natal bundles start at $199 (even then, still highly questionable without a killer app), or MS offers something completely unexpected like Halo Reach with Natal controls in a Natal/Halo Reach/250GB 360 SKU bundle at a giveaway price of $299. But I just don't see either happening.
In about one week, we should all have a better idea on what the Fall lay of the land should look like.
