As long as the pin outs and chip config of the ram sticks are the same, faster ram on a slower system should be no problem. The faster ram will have to run no faster than the slow ram can tolerate.
Crucial has an excellent memory selector tool... you can plug in your system details and it will lead you to the proper ram for your system... even if you are not planning on buying Crucial ram, you will at least see exactly what type of ram your system can take.
http://www.crucial.com/