I want to say that back in the day you might have gotten away with this but probably not doable. IIRC the memory controller was on the northbridge or maybe seperate entirely. This is a cloudy recollection but in the days of FSB I believe it was kind of run seperately in the eyes of the mobo. Now-a-days both AMD and Intel memory controller is on the CPU. All of which doesn't really matter cause you're talking about 256gb of ram which would obviously have to be newer.

What app needs 256gb?