Interesting. I had a 650 but never heard anyone using 64MB SIMM - successfully. The 6100 could. As far as 72-pin SIMMs, has anyone even SEEN a 128MB? System 7 had a limit of 128MB of addressable memory unless VM was on. Some people were able to use a RAM DISK to overcome limit and address more than 128MB (with 512MB installed).
I had QuadDoubler, and though it isn't cost effective, and now Sonnet Presto PPC is available, but a 7500 would be better use of $$$ probably.