When Intel decided to officially implement a 100 MHz system bus speed, they understood that most of the SDRAM modules available at that time would not operate properly above 83 MHz. In order to bring some semblance of order to the marketplace, Intel introduced the PC100 specification as a guideline to manufacturers for building modules that would function properly on their upcoming i440BX. With the PC100 specification, Intel laid out a number of guidelines for trace lengths, trace widths and spacing, number of PCB layers, EEPROM programming specs, etc.
There is still quite a bit of confusion regarding what a 'true' PC100 module actually consists of. Unfortunately, there are quite a few modules being sold today as PC100, yet do not operate reliably at 100 MHz. While the chip speed rating is used most often to determine the overall performance of the chip, a number of other timings are very important. tRCD (RAS to CAS Delay), tRP (RAS precharge time) and CAS Latency all play a role in determining the fastest bus speed the module will operate on to still achieve a 4-1-1-1 timing.
PC100 SDRAM on a 100 MHz (or faster) system bus will provide a performance boost for Socket 7 systems of between 10% and 15%, since the L2 cache is running at system bus speed. Pentium II systems will not see as big a boost, because the L2 cache is running at ½ processor speed anyway, with the exception of the cacheless Celeron chips of course.