Originally posted by Z-man:
just to follow up on what I found out here and other places. RAM seems to be backwards compatable, provided is it the same type...SDRAM, DDR, DDR II.... You can use a faster RAM in a slower machine and the RAM will just clock to the slower speed. In my case, I had RAM at 533 mghz, and put it in a machine designed to use 400 mghz. It runs just fine, but clocks at 400 mghz. In some cases in might not matter, like with 100 SDRAM and 133 SDRAM. I know these are basically the same and will run in any machine designed for either. The 133 just gives you a performance boost.
That's pretty much true. Actually it depends greatly on the chip and the motherboard.
Newer boards don't care as much, but the DDR and SDRAM boards did.
Depending on the board, if you mix frequencies i.e. you have one stick of 100 and one 133 the 133 chip will step down to 100. These chips are (or were) commonly labeled PC100/133 meaning that they were built to do this..whereas chips labeled PC133 would only run (stable) at that frequency.
Like I say it's not really an issue now except that RAM is so cheap it's kinda silly to buy different frequencies.