256GB will likely work on your X99, but check your board's manual.

Yes, it will almost certainly work, provided your motherboard has enough DIMM slots and supports 32GB RDIMMs. Most X99 boards have eight slots, so 8×32GB = 256GB is physically possible.

The E5-2699 v3 is a Haswell-EP chip, and its memory controller handles registered DDR4 ECC fine. ECC will be enabled automatically with the right sticks. The main hangup is motherboard BIOS support for 32GB modules — some older boards may need a BIOS update to recognize them. Also, be aware of rank limitations: if those 32GB sticks are dual-rank (common), populating all eight slots could push the memory controller’s limit. Most X99 boards are fine with eight ranks per channel, but check your specific model’s manual.

No tricks. No special config. It’ll address all 256GB, assuming the board sees each stick. Just don’t cheap out on the RAM — buy known-good RDIMMs from a reputable seller.

Future You will appreciate not having to think about swapping sticks for a long time.