halo first week sales were about 5x higher than MGS4's so you cannot compare. A 70 drop for Halo 3 would mean 2nd week sales for Halo were better than MGS's first week sales.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut