So this seems to use 6.144TiB as the limit that will fit in RAM. That's 1.536TiB x 4 when using the latest Xeon I could find[1]. According to the specs though you should be able to use 8, so the total limit should actually be 1.536 x 8 = 12.288 TiB. 12TiB of RAM, that's quite amazing.
[1] http://ark.intel.com/products/84688/Intel-Xeon-Processor-E7-...