the way I understood it was you needed the same amount of memory on each channel, they could pair a 128MB chip with a 256MB chip (or do 3 x 128MB) on each of the 64 bit channels but that would net us 1152MB total, seems odd to round that amount down to 1GB for marketing
but just like the strong rumors of the 5770 having a 192bit bus right up until its release, I just don't see it happening for the 550.
its easy to assume that the 550 will have the extra ROPs and memory controller that were disabled on the 450, but its just as likely that 40nm manufacturing was mature enough at the 450's launch to go ahead and have those features enabled then, but were instead disabled for reasons such as the GPU simply being shader limited and the extra ROPs and memory bandwidth unable to provide a tangible benefit