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.