RHEL6 has a theoretical maximum of 48 bit addressing space(at least with the 1st beta), which is 256 TB.

From: http://www.theregister.co.uk/2010/04/21/redhat_rhel6_beta_1/

x86-64 architecture presently allows 256 TB (48-bits) physical addressing, and Itanium and at least some its operating systems (HP-UX, OpenVMS) have 50-bit physical addressing available.

The big HP NUMA boxes are Superdome and Superdome 2, and those have global memory access, though access to the local memory is faster than the remote memory; the requisite non-uniform memory access speeds.

Last I checked, the current Superdome and Superdome 2 series maxed out around two and four terabytes of physical memory.

If the current four terabyte maximum memory configurations and possibly the architectural 50-bit addressing aren't enough and you're willing and able to buy that much physical memory, then HP would probably interested in chatting with you.

No RHEL on Itanium, though.

z/OS :)