I jsut got a PIII/500 and a 128-MB PC-133 DIMM. I also got an Alpha P3-125 heatsink with twin fans. Anyway, I took out my 2 64MB PC-100 DIMM's and put the new one in. Because of the size of the Alpha, I couldn't put it in the first socket so I put it in the third. I booted the system up with all the settings at normal for no overclocking. Things seemed to work fine, but when I tried to restart the system, the light on the monitor went from green to amber and nothing else happened. It would only restart if I cut off power to the system. Pressing reset or the power button didn't work.
After the system restarted, I tried the restart again. Sometimes it worked, sometimes it didn't When it didn't, sometimes the monitor turned off, sometimes it stayed on but displayed a no signal message.
Even though I started at the default 100 FSB, I tried different FSB and voltage settings to see what happened. It would usually boot and start Windows at up to 122, but 124 and above produced various amounts of instability.
I then tried moving the DIMM to the 4th slot. I tried all the same things I did before, but there was no difference.
Finally, I took out the new PC-133 DIMM and reinstalled the 2 64MB PC-100 DIMM's in the 3rd and 4th slots. Now I had no problems. I started in un-overclocked mode and restarted the computer from Windows about a dozen times. My overclocking limit is still 122, but I'm running at 120.
The only thing I can think of now that I didn't try earlier was changing the CAS latency setting. Originally, it was set at 3. When I installed the PC-133 DIMM and restarted, the bootup screen said I should use the setting of 2 for better performance. I switched it right away, and I didn't think to switch it back to see if it was the problem. I switched it back after changing back to the old DIMM's, again because the bootup screen suggested it.
Since it doesn't seem to affect my overclocking ability, I may as well just leave the old DIMM's in there. That actually gives me an advantage: My parents have a computer with a single 128 MB PC-100 DIMM, so I can take that out, put the PC-133 in their system, and put their DIMM in my machine, giving me 256MB of RAM.