Enable Fast Writes on AMD64 Systems?

[Bovinicus]

It's hard to compete with all these Doom3 threads circulating, but I thought I would pose this question anyway. I recently updated to the newest Omega version of the ATi Catalyst drivers. When installing them I was prompted to enable or disable fast writes. The recommendation was to disable it barring two exceptions. One exception was if the system was an AMD64 system. Does anyone have any insight as to why this might be? I assume it's related to the on-die memory controller, but I'm really curious.