I've found older SDRAM sources with lower speed grades (133Mhz) are getting more difficult to find, while the higher speed grades (166Mhz+) are more readily available.
Not being a memory expert, I'm not sure if the speed difference is just related to access times inside the chip or if it has anything to do with the data output edge rates.
If, for example, I have a board that works with 133Mhz SDRAM and meets radiated emission limits, should I be concerned about switching over to a 166Mhz speed grade chip (even though I won't be changing the bus speed)?