Using SIMMs that are faster than a computer's rating should not cause any
problems. It's like arriving at the bus stop 10 minutes early--you'll just be
there with time to spare but it won't make the bus arrive any earlier. In the
same way, your data will be ready for the CPU faster, but the CPU won't read it
until it's ready.
As long as your SIMMs are good quality and the proper resistors have been
clipped, there should be no problem using faster speeds.