I think the OP is talking about the 32 bit barrier you hit when you install more than 3GB of ram because of the system not being able to address more memory than that. I don't know if it applies to macs, but it does on the windows end.
It's talked about more
here
First of all, there is no 32 bit barrier in the Mac OSX (otherwise, how could Mac Pros and G5's have been addressing 8 Gb + of RAM for years?). Second, it is not an issue with the CPU having a 32 bit barrier, because the intel Core and G5 chips are 64 bit.
It is a problem with the specific chipset of the first generations of intel Core motherboards, which cannot remap addresses for system resources like PCI slots and other hardware addresses out of the 4 Gb address space. These hardware resources don't use RAM, but they do use the addresses that the RAM would need, so RAM at those addresses cannot be reached. As mentioned, the Santa Rosa chipset does map the hardware addresses out, and therefore can address 4 Gb of RAM.