Just as ATI claimed that the R520 was 512 bit capable, at least internally, it will do the same with the R600 card. R600 supports 1024 bit internally or as Joe Macri likes to calls it "internal ring bus marchitecture".
This is pure marketing, as physically the card has a 512 bit memory controller and that's about it. 1024 is a fictive number for the intercommunication between the chips inside.
The R600 is made of four blocks and the good thing is that the R600 memory wise can support DDR3 or 4 and even older DDR2 is possible.
Even the RV630 and 610 have the same memory controller, at least conceptually, but with less memory nodes so to say. Realistically RV630 and RV610 only supports 128 bit, as that is how ATI wants to market them.