Variety are reporting that, according to "sources close to publisher Take-Two" (ie most likely the guys in charge of selling the game to retailers), Grand Theft Auto IV is poised to take in around $US 400 million during its first week on sale. Yowzers. To put that in perspective, Halo 3's launch took in a paltry $US 300 million.
To put it in further perspective, Spiderman 3 and the third Pirates movie hold the record for the two biggest worldwide opening weekends in cinema history, at $US 380 and $US 340 million respectively. So, yes, we're talking batshit crazy numbers here. They're also reporting that, based on discussions with retailers, the 360 version is easily outselling the PS3 one, though with both a higher install base and the promise of DLC, that's not the biggest shock in world news today.
'Grand' profits [Variety]
Grand Theft Auto IV on Xbox 360 selling better than Playstation 3 [Variety]