I'll give Gears 2 6.5m-7m lifetime(AWESOME, to be sure, but not as optimistic as yours.) So, yeah, it'll outsell it's original, but I'm just not ready to put it at the Halo level yet. Halo, so far, is the second most important game this gen for the HD consoles(saleswise, with GTA4 being the most.) Most people will be buying it for the online, but if you want it to sell at Halo levels, they're going to have to nail the single player or many folks will just rent and forget.
*I'm not saying Halo3 had the greatest single player, I really don't know. But the Halo series is a brand defining franchise and I think it'll take more than online chainsaw fights and meatshields to unseat the king(of 360) from his throne.







