In a recent Asked & Answered post on their blog, Rockstar has today confirmed that Grand Theft Auto 5 will ship with two discs for the Xbox 360, with a mandatory installation on the Xbox 360 and PS3, however the PS3 version will only include a single disc.
On the Xbox 360, you will be required to install the entirety of Disc 1 onto the console, with Disc 2 used to play the actual game, this will mean you will need at least 8GB of free space to install, but Rockstar have confirmed that you can use a USB 2.0 thumb drive with a minimum of 15mb/s read speeds.
On the PS3 however, you simply insert the one disc and it will begin installing the 8GB of data.
GTA 5 is currently expected to launch on the Xbox 360 and PS3 on September 17th, but there’s still no news as to when it will reach the PC or next-gen consoles.
Source: Rockstar blog