HD will dramatically increase GTA4 sales

more it is real, more occidentale male teenager will buy it !

No question about that, but if that's the only market interested, and most are too young to buy it and parents deny them, then that's definately a huge hit to sales.

GTA always has a red and very visible "18" on it, and there's probably a reason for it. I would not let my kids play GTA if they weren't even in their teens yet, not because I think they'd go on a rampage and kill 500 people, but because its unneccessary when its clearly labelled and leveled at a more mature audience.
So, with that in mind, I don't think HD will hurt the sales since most buyers probably are over 12 years old by default.
Also, GTA's violent nature is brought forth by whomever plays it, it was not made solely with havoc and mayhem in mind, that is rather a conseqeunce of different ways of playing the games.
Rockstar probably made these games this way so consequences would seem real and tangible, with the hazzle it would likely be being chased by the police in a buzzling city full of civilians.