The reason I state that it will be a over 100k boost is this. Let's break down some numbers.
I work at 3 separate Gamestop locations but about 80% of the time I'm at my main store. My main store currently has roughly 32 pre-ordered Halo Reach console bundles. The other two are less with one having around 25 and the other right at 20.
So let's assume that that if company wide the stores are only pre-ordering HALF of what my store is doing that alone would still be a 105,000 boost and thats just from Gamestop, not counting ANY sales from other big retailers like Wal-Mart and Amazon. Now lets be a little more generous and say that company wide our 7,000 stores are closer to my other two stores I work at and hovering around 20-25. That would be a boost of 175,000 units from Gamestop alone. Simply visit the Bungie forums and look at how many people have held off upgrading their systems to the Slim because of the Reach bundle.
Halo Reach is defiantly be a hardware pusher no doubt.