It's impossible to ever really know the extent of piracy on the 360 but it would be interesting to know how many copies of GTAIV will be downloaded rather than purchased.

well, all major games are torrented onto the web, and it doesn't seem to have a noticeable effect on sales, i feel that this won't be an exception.

 You really have no way of knowing that.  The only way we would know if it effected sales is if we had numbers in a world without illegal downloads.  And we don't.  So for all we know, without piracy, the gaming industry would sell much more than its selling now.  

 I agree with both of you somewhat, on one hand you got the people that would have never bought the game in the first place downloading it, then you got those that might buy it then download it and have enough respect for the people that worked hard to make the game and they go out and purchase the game, when they might not have if they hadn't downloaded it, then you got the other kind of people that hurt the business that flat out steal the game and never have intentions of buying it.

The number will be statistically equivalent to 0. Really, the vast majority of pirates would not have purchased the game legitimately in the first place, and piracy is inherently taken into account when designing a game. It's simply something that would exist regardless, for as long as there are people who can't easily make ends meet.

Besides, piracy is about the only way people could consider buying the machines in the first place in some areas of the world, because that alone will cost them more than many of us would spend on the system, accessories, and games combined over the system's lifetime.