The up and coming R600 will have a real 512-bit memory controller. Unlike its predecessors which had an internal 512-bit ring memory bus, the R600 will have it externally as well. 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-bit memory ring turns to be the real thing, we are talking about 128GB/s of memory bandwidth with GDDR4 clocked at 2000MHz. We also learned that the R600 core may use memory faster than 2000MHz as it will be available by first quarter of 2007. If ATI keeps pushing the chip we might get even faster GDDR4 chips at production time. - The Inquirer

ATI's R600 chip has an external 512-bit memory bus

Even the PCB of the R600 based cards 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 chip, but yet again it will come at least three months after NVIDIA. The G80's memory works at 384-bit as NVIDIA pretty much dis-unified everything in G80 from shaders to memory controllers. NVIDIA likes to make rules and probably could not get more than 384-bit wide controller in the chip, as the G80 core is still a 90 nanometre chip. It's a shame that we will need to wait at least until February to see it in action.