You have Sony fanboys saying laughable numbers in this thread for obvious reasons.
Halo Wars will sell at least 3 million in it's first year.
* like the OP said, user base of X360 when Halo Wars is released is massive compared to C&C release.
* C&C wasn't exlusive to X360 platform. These 2 factors alone will secure Halo Wars will sell 1 million.
* Halo-fans will try RTS-gaming for real thanks to this game. When Halo Wars is released, there will be 8 million Halo 3 sold. At least 1/5 will of these buy Halo Wars (1.6 mill). ---> 2.5 million.
* Christmas release is the best time.
* Ensemble studios is the dev. Their Age of Empires series sell in the 6-10 millions for each release. Ensemble make +90% games, top notch AAA titles.
* These two above factors plus big marketing could mean another 1 mill.
Conclusion: most likely around 3.5 million sold world-wide in it's first year.