For all hypothetical purposes it COULD outsell it, as the userbase is larger, but the amount of press on Killzone 2 is staggering, it's had huge amount of press and hype since 2005, The Conduit showed up not even a year ago and only been shown off at 1 E3, an E3 that was small and outside of the press for FFXIII not much big to get everyone's attention.
So most likely no, but is it a possibility, maybe, many Wii fans are looking forward to it, CoD:W@W will be hitting 1million, so it would easily get 1million I'd say, but 2million+ would be a stretch, while I think with the hype Killzone will get at least 2million, though as bardicverse said it could bomb if people don't like how it plays, even games rated very high by reviewers and some PS3 fans doesn't mean the masses will like it and word of mouth could be very negative, GTAIV could have possibly done a lot better then it has if it wasn't for the negative word of mouth.
MaxwellGT2000 - "Does the amount of times you beat it count towards how hardcore you are?"
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