Well, you ask a complex question in a simple manner.
Let's make some assumptions. Assuming that either you have unlocked the Duron and can run it at a lower multiplier (i.e. a Duron 800 with the multiplier lowered to 6x but the FSB raised to 133) or a good Duron which is capable of a 1/3 overclock, like a Duron 700 staying at the default 7x but running at 133 for a speed of 931.
Either way, the simple answer to your question is that the Duron runs fine at 133 so long as you have a chip that you can make run within it's own speed limitations.
Joe