"But everything else works!" must be a favoured line of ATI users.

This is nothing whatsoever to do with ATI but yet everything to do with them.
All other games use static images for their textures. These might be images at, say, 256x256 or 512x512 resolution. You take an image of some brick, repeat it a few times across a surface, and you get a brick wall. You load the image once during startup and it stays in video RAM where it can be quickly accessed as required. Everything from Quake to BF3 does this and the downside of it is that you get a very obvious repeating pattern. Some more modern games try various tricks to hide it, but it's still there.
Rage doesn't do this. Rage doesn't have repeating textures, everything you see in the game is completely unique and non-repeating.
Now, this is obviously stupid because there's no way you can do it; you'd need a 128000x128000 texture to do it, current hardware just does not support textures that size and is not likely to in any kind of short or medium term, if ever. But yet Rage does it.
The trick is that it dynamically loads portions of this huge image file (which answers the "why is Rage so big?" question) into a smaller texture on your video hardware - 4096x4096 or 8192x8192. In order to load this at some kind of playable framerate it first loads in lower quality versions of the image portion, then progressivly loads in higher quality versions until you get the full thing. If it can load them in fast enough you don't even notice it happen; if it can't - that's the dreaded Rage texture pop-in.
Here's where we start having fun.
NVIDIA can load them fast enough.
Intel can load them almost fast enough.
ATI on the other hand
svcks for updating dynamic textures at runtime. Yes, they're good (often better than NVIDIA) if you hit them with purely static content, but for some reason, when they are faced with dynamic content (particularly textures) they positively svck.
This is not a Rage problem; this problem manifests on ATI hardware all the way back to Quake; it can be tested and confirmed all the way back to Quake, you can write your own benchmarking program that runs on all 3 vendor's hardware and confirm it. ATI loads dynamic textures at runtime at half the speed of Intel and quarter the speed of NVIDIA.
Megatexture giveth and megatexture taketh away. Just like repeating textures, but in different ways.