The last two Halo's proved that Halo is no longer a system seller, at least for current HD generation. I think it would be tough to assume that Microsoft will be sucessfull in 2010 based on only Halo. They need more to compete with Sony's AAA titles:
-- GT5
-- God of War III
-- Heavy Rain
-- MAG
-- The Agent (from Rockstar)
-- The Last Guardian
-- ModNation Racers (This will be huge).
-- White Knight Chronicles.