I recently bought a 128MB PC-133 stick. I used that one and then inserted an old PC-100 32MB stick. I benchmarked it (just Norton Utilities' System Information) and my total score dropped about 8 points from when my PC had 128MB at PC-133 to to when it had 160MB at PC-100.
If your PC-100 stick is not much (in terms of size) compared to the PC-133 sticks, then I guess that you should just use the PC-133 sticks. But if the PC-100 sticks are large enough, then more probably than not you'll get a performance boost even when the PC-133's are degraded to PC-100's.