I've run 128 meg DIMMs in many PCI Macs including a 7300 with 1 gig and a 9600 with 1.5 gigs for many years. No problems, so long as you get the right DIMMs. Mine were from Other World Computing, so they're correct and have a lifetime warranty.
I have successfully run 128Mb DIMMs in my PM9600 for years, and also successfully used them in the 7300/7500/7600 machines and I beleive they worked in my PM5500 also. that covers most of the major board architectures that use 5V 168 pin DIMM's.