I'm not 100 sure, but it seems that the 7DXR is running with 3 sticks un-registered DDR memory, at least for the 100 or so minutes I had the 3 sockets populated. You can use only up 2 Giga un-registered, un-buffered memory. There is, by the way, nothing about this explained in the 7DXR manual. On page 3 they simply
"supports up to 3 Giga". But on their webside, Giga is explaining that you you can use a maximum og 2 Giga memory unbuffered, means unregistered, and up to 3 Giga buffered, means registered memory.
I was not aware of this point, so I just made a quick check as to see whether all 3 sockets were working, when I wrote my review. I put in
2 x 128 Samsung PC 2100, unregistered, unbuffered
1 x 256 Nanya CL 2.0, unregistered, unbuffered
Unfortunately I sold the 2 x 128 sticks after the review, so I cannot check it anymore. But other users in the amdmb.com Gigabyte forum reporting the same.