black ops first week was 20% more than mw2.
modern warfare 3 first week was 30% more than black ops
So black ops 2 will sell 9mil first week if it increases by 20%, 9.75mil if it increases by 30%.
MW3 user metacritic scores were very significantly lower than MW2 and Black Ops. So either the fans are fed up with the series or lots of trolls, or probably both, but I do think cod sales reached a tipping point with mw3 at least in the USA, and will not be able to match mw3 sales.
As for halo 4 and black ops 2 launching near its other. I really think this will hurt the first week sales of both games. MW3(360) may have doubled halo reach's first week sales, but mainline halo games are the second best selling console games in terms of first week single platform. You can think of it this way. MW3 only managed to double halo reach first week, while it nearly tripled or more every other big console release's first week. I think halo 4 could sell 4mil first week, and black ops 2 6.8mil first week.
A thing to note halo 4 selling 4mil first week in november is signifcantly worse than halo reach/halo 3 selling 3.8mil first week in september. November means holiday boost is already here, so halo 4 is experiencing its "bump" in sales at launch, while halo 3 and reach got theirs a few months after launch.