How to check if system supports 64-bit PCIe decoding?

Was it manufactured in the last 10-15 years? If so, it likely supports 64-bit PCI decoding. 64-bit PCI, PCI-X and PCI Express buses all support 64-bit decoding.

You can verify 64-bit decoding is in use with a Linux command like:

sudo lspci -v | grep "Memory.*64-bit"

While this BIOS option is disabled, you will see that all of the memory has been decoded below the 4GiB boundary. If you see no output, then no 64-bit memory decoding has been done.

BTW, you should leave this BIOS option disabled unless you have PCI Express device configurations which require it, such as multiple installed GPGPUs. Some older Linux kernels might not boot with it enabled, as well, so you may need to update your OS before enabling this option.