X... what IS the diff with an X?

Quad core graphics sub processor

 

I believe it's just the GPU portion of the die is quad-core, and the memory is moved off-die and onto the rest of the PCB. Where an A5 has dual core CPU and GPU with 512MB RAM built-in, A5X has dual core CPU and a quad core GPU, with 1GB RAM elsewhere on the PCB.

One could expect A6X to be has fast as the iPhone 5, but with higher-end graphics and externalized RAM.

 

Okay. Even though it's information that you can easily disseminate from the Googles, I'll put it here.

A5X = A5 CPU + 2*A5 GPU

A#X = A# CPU + 2*A# GPU