According to Variety, they are saying that sources close to Take Two the parent company of Rockstar Games, the publishing arm of GTA IV, they approximating that they will sell $500 million or 6 million units the first week of the April 29th release date.

Normally publishers can be close to accurate numbers by the amount of units already ordered, but they rarely released those numbers.

The record for one week sales was last year, with the release of Halo 3 for the Xbox 360 at $300 million and 5 million units. Since GTA IV is for both the 360 and PS 3, the record is expected to fall.

Source