Sigh...
Ok, look. On a PC-133 system, you can use either PC-133, or PC-100. If you run PC-133 this will ensure normal operation and keep the system bus at 133MHz. If you run PC-100 then the bus will drop to the highest speed of the slowest stick. In other words you drop the entire system bus from 133MHz to 100MHz. This is not good. Buy better RAM.
On a PC-100 system you can still run either module. Although some older systems have problems recognizing high density modules. Feel free to mix and match PC-100 and PC-133 sticks. If you stick a PC-66 stick in there, the same rule applies. The system bus will drop.
On a PC-66 system (G3's, PII, Celeron, K6, K5, etc.) you can run any damned module you want. There is nothing slower than PC-66 so go for it. Keep in mind though that the sticks must be SDRAM modules and not SIMMS. There are PC-66 SIMMS out there for a few franken-motherboards that appeared for a short time.
Ok, look. On a PC-133 system, you can use either PC-133, or PC-100. If you run PC-133 this will ensure normal operation and keep the system bus at 133MHz. If you run PC-100 then the bus will drop to the highest speed of the slowest stick. In other words you drop the entire system bus from 133MHz to 100MHz. This is not good. Buy better RAM.
On a PC-100 system you can still run either module. Although some older systems have problems recognizing high density modules. Feel free to mix and match PC-100 and PC-133 sticks. If you stick a PC-66 stick in there, the same rule applies. The system bus will drop.
On a PC-66 system (G3's, PII, Celeron, K6, K5, etc.) you can run any damned module you want. There is nothing slower than PC-66 so go for it. Keep in mind though that the sticks must be SDRAM modules and not SIMMS. There are PC-66 SIMMS out there for a few franken-motherboards that appeared for a short time.