800xa 64 and 32 bits
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Starting with 5.1 RevB all 800xA media kits are "unisex" and will fit both 32 and 64 bit architectures.
From this revision, you are free to mix 32 and 64 as much as you like in the same system.
To be honest, *clients* and *engineering nodes* are often in more need of 64 bit OS and ample amounts of RAM and CPU GHz (not cores!) to be able to host a large workplace with multiple monitors and lots of graphics plus engineering tools. Servers often run lean on CPU and RAM.
64-bit machines should be configured with 8 GB RAM or more.
"Go large" in clients
"Go medium/low" in servers*
*) Some application servers will benefit from more powerful hardware.
Check the release notes and installation guide for the specific version you are installing. You will find detailed instructions for which versions of Windows can be used for each station type. Yes, it's possible in most (but not all) cases.
But tbh, its not a great idea. It doesn't cost any more to use Win 64 and you will appreciate the ability to use the extra memory even if you start with only 4GB. Trying to save a few dollars on client machines now is going to hurt later when you have to upgrade them.