I have not heard of ACPI interfering with the big memory option,
however many motherboards have issues with ACPI and the kernel's support.
To test your other kernels, you can just add "noacpi" to the boot parameters from lilo/grub boot prompt, whichever you use.
I suspect yours is one of the mobos with a little buggy acpi bios.
Congrats for finally getting it working, though.