It looks like Dell doesn't sell 2RX4 8GB RAM modules any more. They recommend 2RX8 and 1Rx4 for the R720XD.
Can I mix and match my old 2RX4 with the new 2RX8? I believe the manual says no, but it seemed like an off-hand recommendation, and not a big bold warning.
I have 12 modules installed.
DUAL IN-LINE MEMORY MODULE, 8GB, 1600, 2RX4, 2G, DDR3L, R