The upcoming ATI R600 GPU will have a real 512 bit memory controller.
This means that the packaging of the chip will be extremely expensive. The wider memory bus you use the more pins you need in your chip package.
If the 512 memory ring turns to be the real thing, we are talking about 128 GB/s of memory bandwidth with GDDR4 clocked at 2000MHz. We also learned that the R600 may use memory faster than 2000MHz as it will be available by Q1. If ATI keeps pushing the chip we might get even faster GDDR4 chips at production time.
Even the PCB of the R600 will be super complicated, as you need a lot of wires to make 512 bit memory to work. Overall it has the potential to beat Nvidia's G80, but yet again it will come at least three months after Nvidia.