I think it will be close. If you like Reach then you probably have halo 3.
I can see it passing it though, as there are more consoles out there now than 3 years ago, as well it's not really a "sequel" so new people who haven't played the Halo franchise can probably pick this up and not feel "lost" playing it.
Also there are probably (too bad these numbers aren't tracked) a lot of people who traded in Halo 3/ODST and was bought by someone else, and I think both these people will buy Reach new if Bungie can deliver on what they are saying the game will look like, I think this will be a Day 1 for most.
Unicorns ARE real - They are just fat, grey and called Rhinos








