This is my prediction looking based on the amount of 360's out there and selling at the moment.
Gears 1 sold 600,000 with 5 million 360's on the market
http://vgchartz.com/weekly.php?date=39033&console=&maker=&boxartz=1
Halo3 did 3.8 mill with about 12 mil 360's GEEEZZ!!
http://www.vgchartz.com/weekly.php?date=39355&console=&maker=&boxartz=1
Preorders as of 10/15 were third to Halo 3 and Halo 2 witch the latter it probably surpassed by now. It will be hard to tell if it gives a hardware boost, but I think it will probably about the size boost that MGS4 gave to the PS3.
Any thought