Halo 3 will sell a lot of copies, Microsoft is openly predicting first dale sales of over 2.5 million and given they have incentives to underestimate that number you could easily see like 2.75 million first day. And there is no reason to expect it to be less than quality. But its hardly going to push how people view games which is why it might win a bunch of fan choice awards but up against Bioshock (changing how shooters work), COD4 (much more traditional but very high quality), Mass Effect (huge scope plus revolutionary dialouge) and AC (changing action adventure framework). And that doesn't even go into Nintendo first part titles for the Wii like Mario, Metroid, and Smash Brothers.