Short version is that it usually works, and should work according to the standard.
Long version is that it depends slightly on where you got the memory sticks. To run at a specific speed, the computer needs to know which timings that particular memory chip needs. This is essentially the number of cycles that the computer waits between each step in the read (or write) process. The higher the clockspeed, the more cycles are needed for each step. The standard says that the DIMM should supply the timings for all standard clockspeeds up to and including the one it is rated for. Unfortunately, some manufacturers cheat on this and only submit the timings for the rated speed. If they do that, it will only work at the rated speed.
One way to really know is to plug them in and see if it works. Chances are good that it will work, but it's not certain. If that is for some reason not an option, there are tools that will let you read out the memory timings on all DIMMs currently installed. Don't have a link handy, but I'm sure you can Google it up if you need it. If not, let us know.
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