Older motherboards (particularly DDR based systems) can be VERY picky when it comes to memory, especially when mixing memory.
If your system defaults to 333, I would suggest you let it and leave it alone. You will see virtually no difference from a user perspective whether it is running at 333 or 400.
Also, if possible, use only matched DDR modules in a system. If you use different brands, or even different models of the same brand, the motherboard will detect the differences and will set the speed to the lower setting to optimize stability (not performance).
If you mix slower and faster DDR modules, the system will set to the lower speed as well (least common denominator).
Bottom line, I recommend you go with the 333 setting and match your memory as closely as possible to see if that helps. Good luck!