1:1 doesn't mean ANYTHING AT ALL as far as A64 systems are concerned. Know that, and never differ.
The reason is that, unlike Intel systems, in A64 systems, there is no FSB. There is the HTT. If there was a FSB, then you would have to run the FSB/MEM ratio at 1:1 because if you didn't, there would be a lag in the communication times between the 2. But because there is no FSB in A64 systems, and because the HTT is not an actual bus (its an artificial clock used to get the CPU Speed) there isn't even any such thing as a 1:1 divider in A64. You do not benefit from running 1:1 against any other ratio. Or you could think of it as, you don't lose any performance from running away from 1:1 ratio.
The only reason you would have to run 1:1 is if you have memory that can support higher clock speeds, and you want it to run faster so you get more bandwidth. But if you memory doesn't OC that far (Corsair ValueSelect) then you can set the divider to 6:5 or 3:2. They will run your memory at a slower speed so you can overclock your CPU farther.
Thats all you should need to know on that topic. I started 2 HUGE threads on the topic on both XtremeSystems and OC Forums, and the conclusion was the same on both.