Halo: ODST (which is sorta an expansion, and typically called that by fanboys for console war fodder) sold 2.2m 1st week. Halo 2 & 3 both broke 3.3m 1st week.
Mario galaxy does not stand a chance as it will probably only get ~1m, so no reason to discuss this.
Gran Turismo did 2.5m for GT4. Other games don't have weekly sales, but I have no reason to believe that other games did much more than that. Maybe 3m 1st week for GT3. However, GT3 sold more in Japan than GT4 with a smaller 1st week, so maybe the hype for GT4 was higher because of GT3, so we really don't know if GT3 sold more 1st week.
the Halo series has shown it can pull +3m 1st weeks for main games, and I doubt that trend is going to stop. While not having full information on GT it seems the sales tend to be sustained over a longer period of time. So I think GT can do over 2m 1st week, while Halo will do over 3m.
Winner Halo:Reach.








