Most games are available on the net before they are in the stores, and it really doesn't have any effect on the sales provided it's a quality product. There are people who will always buy the game, people who will always download the game, and there is a small group that in between who will usually buy a game as long as it's worth it. Nothing will change this. Having it released on the net a week before it's in the the stores isn't going to make people who would usually buy it decide not to, just as it being pirated a week late wouldn't stop the people who are going to download it.