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I was trying to find any reference in the following Dell* site: https://www.dell.com/en-us/shop/dell-desktop-computers/xps-tower/spd/xps-8930-desktop, however there is no mention about the support for the Intel® Optane™ Memory 32GB version, although there is a clear note about the support for the 16GB version.
The best recommendation indeed is to contact Dell about this concern. They will be able to clear out your doubt. In the site above there is a section of questions/answer by the users that you may find useful to post this concern.
If there is anything else we can help you with, feel free to ask.
Have a nice day.