It's not gonna beat Halo 3 at anything, saleswise. Reasons:
1. A lot of buyers were disappointed with H3 and thus are unlikely to feel the need to rush and buy its expansion.
2. Even a lot of people who like the game aren't going to pay $30-$40 for a couple new maps and a 2-3-hour campaign.
3. This will see a lot of marketing, but nowhere near the Hal-Mountain-Dew levels of the original.







