If you're getting GTA 5 on Xbox 360 tomorrow, don't install the play disc to your hard drive. Rockstar issued the recommendation on its Twitter account today.
@mfw For optimal performance, we recommend not installing that disc. We will have more info on our Support Site at launch.September 16, 2013
The Xbox 360 version has a disc for installing and a disc for playing. If you get too excited about this whole "installing" thing and do it for both, you may find your performance suffers. Rockstar didn't say why, but that it will have more info on its support site after launch.
Both the PS3 and Xbox 360 versions require an 8 GB install from the install disc to play, in case you need to clear up some space.