Can I mix PC100 and PC133 SDRAM SIMMs?

As magarity has stated is correct just make certain that the SIMM/DIMM's are not mixed between registered and unregistered, and that they will run at the PC100 speed no extra benefit whatsoever for the PC133's

Also one side note do not mix non-ECC and ECC memory from PC100 to PC133, they did slightly change the way it handles the error checking and it has been known to cause minor problems.

Other than that is safe to do it.

"We definitely need a FAQ for this."  Why? You don't want the points!  Anyway, yes, yes, yes unless your system is not able to handle the type of RAM it is, this was already mentioned but someone is sure to say, "NO!".  I have run PC133 Ram on systems that should have been using PC-66 just to prove the point a few times.

I have lately come across a caveat to mixing speeds, though it doesn't apply to most normal usage. Sometimes it will happen that you have 8ns PC-100 memory in a machine with a tunable FSB, and it will work great at 115 even 124Mhz, pass any kind of test you can throw at it, identical PC-100 modules will work together, but put a PC-133 module in that system and it may start throwing errors. Similar things may happen with 10ns PC-66 modules at 75 or 83 mhz mixed with PC-100. May be some wierd kind of RF noise interplay thing, if they're both making it, it works, if only one is, the other don't like it. But this is all when they are out of spec anyway.

Yes it is possible, but some main-boards have some trouble with it. If it is not functioning, try to put the 100mhz in the first mem-slots, and the 133mhz after that.

Thanks magarity, and everyone else who commented.