I know 6 people who already have gta iv on pirated copy for 360.  could this damage total sales of the game on 360 or not. what do you think

No. Most GTAIV players will be casual players (Madden style casuals, Wii style casuals) and they typically are not as technically proficient as more hardcore or PC gamers.

I already know two people who have downloaded the torrent. The game is really good so I can't see people not downloading it when they have had a go.

No, you can't even play the game online if you don't have the original. It'll sell in boatloads.

I think the ban hammer of Microsoft will wipe out those who decide to pirate the game. I thought I read something about them being watchful over achievements from people who've pirated the game.

It'll be the 'Hitler gets banned' video all over again o_0
Stop! Drop! and Roll!

Can't play online, can't get the DLC (I think), can't use for achievements. I just think there would be too many things missing from downloading the game for any decent number to take advantage of the D/L.

It'll have little effect on the overall sales.

 I agree, it might be popular with people who usually pirate, but I'm doubting that sales are going to be defeated by this leak. 

Good thing you can't pirate a blu ray....yet!

And, does anyone know if the only reason that they can monitor is because the game isn't out yet? What about after the 28th? How will they know who pirated the game then?

This wil not effect the sales at all imo