GeNetiX, Halo 3 (2007) will come out before Halo Wars (2008).
Halo 3 is Bungie's last game.
Halo Wars is Ensemble's first Halo game. I have a feeling there will be another as the story events of the Halo universe beg to be fleshed out. Kinda like Brood War's strory continued that of Starcraft.
As for Live functionality, there is NO WAY an RTS could come out on the 360 and not feature multiplay support over Live. It'd be forgotten in a week, even if it has the Halo name next to it.