that depends on the motherboard. If you mean just upping the speed of the ram to 333Mhz and you are sure the cpu fsb will still be at 266mhz, then that is fine. The best setting for the ram would be "spd". if your motherboard supports this then it will read the eepron chip on the memory and run at it's fastest speed.
But it depends on the motherboard. Post the
make and model of it and someone can advise on the best settings for you.