Rage is a 32-bit application so it can't address more than 4gb of system memory (2gb in user space, 2gb in kernel space) - although I think on a 64-bit version of Windows they both cap at 4gb.
If you're running at a combination of high resolution, high AA and large texture cache you could still be low on memory even with 8gb system and 1.5gb video - those kind of settings will eat memory for breakfast and come back looking for more.
Long story short: Rage, like every other id engine before it, is capable of exceeding the capabilities of hardware that's current at the time of it's release, and of doing so in ways that other games don't (also like every other id engine before it). So don't try to max it out.
You seem to contradict yourself. The game cannot use more than 4GB of RAM, but the engine is capable of exceeding the capabilities of hardware in a way that 8GB is insufficient. It's like saying this game cannot be run, in practice.
Dude, I'm intelligent enough to try to lower the settings before complaining. No, it's not an hardware limitation problem. GPU transcoding worked in the first release of the game, before they removed it for some reason. Now they have re-introduced it and it just don't activate.
An official response to this problem would highly be appreciated, but it seems there is a lack of community managers these days.