It could happen. Gears was awfully popular for a new IP and the sequel will be bigger. It wouldn't surprise me if it sold more than any game on the console that isn't Halo.
On the downside, the game is far more violent than Halo 3, which perhaps limits its audience. While Halo is also violent it typically involves colourful worlds and colourful alients, which perhaps allows it to appeal to a wider audience.
One thing is clear though, Gears II will be huge and nobody does big game launches quite like Microsoft.
Debating with fanboys, its not
all that dissimilar to banging ones
head against a wall