Shouldnt there be another divider in there somewhere? I'm running my A64 @ 2.7GHZ and RAM at 200mhz.
As for your question, I would definately say that you should run 2.6/216. OCing the RAM above 200mhz is going to be pretty useless unless you have tight timings as well, but if you go under 200mhz then your sysytem is gonna be starved off bandwidth.
My $0.02