A 1GB configuration using 4 "stacked" modules presents 128 SDRAM chips to the memory bus, whereas "monolithic" presents only 64. The PowerMac G3 that uses PC-100 modules has a limited ability to handle the loads created by so many chips on its memory bus. Overloading the memory bus can cause problems.
In testing on our in-house systems, we've had 100% reliability with four of our own "stacked" modules built with approved chips. However, idiosyncrasies between machines may mean less than 100% reliability with four "stacked" modules. To increase reliability, Advantage Memory recommends a maximum of 3 "stacked" modules and 1 "monolithic" module when going to the 1GB configuration.
It should be noted that the "stacked" module is designed as a cost control option until the "monolithic" prices reach parity. If you decide to use four "stacked" modules, and get unreliable operation, at least one of the modules may need to be replaced with the "monolithic" and more expensive design.