If you're buying RAGE on the Xbox 360, id Software recommend that you go through with the installation option, even though it's going to be around 21-22GB.
"On the 360 we don't have a partial install option," said John Carmack at this year's QuakeCon. "It's all or nothing, which is kind of unfortunate. It means you have to install 21-22GB of stuff which takes a long time but if you've got it and you play it on the 360 that's the way to go."
Carmack explained that not going through with the installation could lead to latency issues when visiting areas for the first time - something which will also feature in the PS3 version of the game.