Go to http://docs.hp.com/en/hw.html
and look at the items there. The PA-RISC servers that run HP-UX are HP 9000s (multiple models shown there, Superdome is largest, RP8420 is partitionable large, RP3410 is a small PC sized unit.
The Integrity RX ones are the ones that use Itanium (Intel 64 bit).
You can run HP-UX 11iv2 on either Itanium or PA-RISC. For large systems you want to stick with the PA-RISC.
FYI. This site doesn't think HP-UX worthy of its own forum. You'd be better off going to HP's ITRC forums (itrc.hp.com) or subscribing to the Dutchworks Mailing List. You can subsrcibe to the latter by following the instructions at http://hpux.its.tudelft.nl/hppd/admin.html