[From: email@example.com (Cameron L. Spitzer)]
Only the person who designed your mb knows for sure. There is no simple formula related to clock rate. However, most people tell me their 33 MHz mbs' manuals call for 25 ns data store and 20 ns tag store, and their 40 and 50 MHz mbs want 20 ns data store and 15 or 12 ns tag. Tqhe tag has to be faster than data store to make time for the comparator to work. Do not buy a motherboard if you do not know what speed and size of cache RAMs it requires in all its speeds and configurations.
If you're not sure, it doesn't hurt to use faster RAMs than your manual calls for. If your manual says 20 ns for location x and you happen to have 15 ns parts, it's ok to "mix" the speeds. It's ok to "mix" RAMs from more than one manufacturer. However, the faster RAMs will not buy you more performance.