depends if your CPU overclock is being held back by 6400 RAM, and if you think you'll benefit from the extra memory bandwidth. If you have a 9x multi, probably not, if you have a 6x multi you might want that FSB higher than 400.
Given how cheap some 8500 is at the moment, it might be a good idea to get the guaranteed headroom, for the same price as some clockable 6400 chips (such as cellshock, ballistx, etc)