So, in summary, do these old boards/CPUs have a GPU limit?
see if the bios has something like "above 4g decoding". short version is it increases the range of memory the mobo can assign to the cards, which is likely the issue. that bios setting is fairly recent in consumer mobos as having 4+ video cards wasnt very common back then when those mobos came out.
that is correct, do remember that in order to be able to access GPUs this way ALL system drivers must support 64bit address space and this means Windows MUST be installed in UEFI mode.