Of course it will sell well...

It's coming out on PC Too

RTS's and FPS's dont' appeal to the same people, RTS's are more of a niche then mainstream like FPS's so I think Halo Wars will sell less then the main trilogy.

If Halo Wars takes advantage of the fabled wireless motion controller and those controls are good it will do 3-4 million... Otherwise 1-2 million as a straight up RTS on a console.

This will sell more copies on PC than on 360.

1 - 2 million

i believe over 2 millions..

eleventy billion.  yea i know thats a big number but its possible.  but really i know it will actually sell atleast 10

3 million at the max.

It will sell 2.5million minimum.

Its has - Great Brand - Great Developer (AAA) - Its not a console port of a pc game - coming out in Christmas - Possible first use of a possibly great peripheral

I think two factors will be the difference between 2-4 million sales.

How good the supposed Xmote is.
How Well they pull it off.

If both the above go well its an AAA game that will make win another genre completely exclusively for the Xbox360 and pull in a halfmillion customers.