To enable UEFI, if not yet, you should format again and before installation convert the partitions to GPT.I will try to install Windows 8.1 64bit.
But another problem arises, Windows 7 32b I installed with the MB in Legacy mode.
But Windows 8.1 64b requires UEFI mode.
But it will delete all the existing partitions.
To get the best from systems with SSD that make the difference.
I have very good experience with Win 10 64 bit.
Sorry it's not the same for You.
As @Filippo said, 64 bit version is needed to use the whole ram you have now.
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