Both Intel and AMD mainstream lines do not offer chipsets that allow 256 GB of memory. Maybe this will change this fall when they announce their new cpus.
Until then, you need to go with a HEDT chipset. However the Intel one is considered clearly inferior as it's lagging significantly behind AMD's Threadripper. For both the chip and the 256GB of memory, especially if it's DDR5, I would expect to reach the upper bound of the budget you have.
If you also want to get a 6950 XT, expect to pay another $1000-1500. But there are new high end GPUs coming from both AMD and NVIDIA in the fall. Just wait for the dust to settle by the holiday season and make your purchase then.
PS. after googling about it, those HEDT systems are more hard to find and more expensive than I thought. Either wait till the end of the fall or get an 12900 with 128GB.
https://www.pugetsystems.com/blog/20...ocessors-2329/