Since AMD does not support or guarantee ganged mode, they suggest to remove this feature in our bios, that's the story.
According to AMD - two modes in AMD CPU for memory interface:
1. Ganged mode - two memory controllers are tied to work in 128bit mode
2. Un-ganged mode - two memory controllers are working independently, but still can be treated as 128 bit mode.
Un-ganged mode is having better performance (overall), hence AMD suggests using this mode.
The “64 bit” indicates 64bit for each memory controller in un-ganged mode.
64 x 2 still equals to 128. So, in un-ganged mode, nothing is downgraded