According to details from VR-Zone, the upcoming GeForce 8600 will have a 128-bit memory interface:
We are able to confirm at this point of time that GeForce 8600 series will be 128-bit and has 256MB GDDR3 memories onboard. Onto the rumours, We heard there are 2 versions; 8600GT and 8600GS clocked at 650/1800MHz and 550/1400MHz respectively for core and memories. Both come with 64 shader processors.
The NVIDIA GeForce 8600 (G84) is expected before the launch of ATI's Radeon X2x00 (R600).