Are you running 32 bit Windows or 64 bit? Even in the later case some "transition era" chipsets (like many used with Core 2 cpus) can have "issues" with large amounts of RAM. It's long to explain but basically it involves conflicts between trying to make a linear virtual allocation space for system RAM vs. the amount of the 4GB *physical* window that's used for PCI peripherals; Parallel PCI uses a 32 bit model and although PCIe supports 64 bit addressing there's still issues with virtually mapping those devices into a backwards-compatible space. Depending on your platform's PCIe fabric and what OS support you have for PCIe address routing going above 3GB of memory or so can force drivers to, for instance, disable DMA transfers, because part of the physical memory space isn't directly reachable to peripherals on the bus.
(If you look it up you can find some incredibly headache-inducing discussions about PCI Express chipset designs; many early/low end chipsets were *basically* still PCI chipsets with PCIe just sort of grafted on, while higher end/server chipsets will have more sophisticated hardware that can alleviate some of the problems.)
In any case, none of this applies to PowerPC Macs other than *possibly* G5s.