As title.
I know that they are working on a 64bit exe, what is the improvements on this exe?

The program can use more memory, and some things can be executed more quickly.

The application can address up to 16 TB of RAM while a 32-bit application can only addres 4 GB of RAM (though practically either 2 GB or 3 GB, because of a ton of conditions)

Basically, when the game has 2 or 3 GB available memory, and some of it is addressed by the game, if you have a high end video card it is possible you will not be able to use all the VRAM on the graphics card.

is there really going to be a bigger update then the first where we get sharper textures?

Meh. I have only 4 terabytes of RAM installed..

what is the sense of using more ram on this game if it runs maxed out with a single card @60FPS?

For modding tools more ram is often very nice to have.
For the actual game having a 64bit.exe doesn't really make much of a difference.

As CopyCat said, it's really only beneficial for modding tools or making the game (or other games with id Tech 5). RAGE has eg. all the textures stored into a single giant texture. They needed a server with 192 GB RAM to combine all the textures into the megatexture.

If a 64-bit patch was to be released for RAGE, I don't think that more than 1% of the PC users would see a benefit from it.

Maxed out means something different for Rage. You can max out the settings, but if you don't have enough vram it will reduce the textures to whatever is necessary to maintain 60fps. Exactly how much ram, vram, cpu cores, etc. the high texture pack might require is anyone's guess.