The CPU attached to my system is W3565. According to Intel ark web site, supported maximum memory capacity of it is 24BG.
(http://ark.intel.com/products/39721/Intel-Xeon-Processor-W3565-8M-Cache-3_20-GHz-4_80-GTs-Intel-QPI)
However, before I decided to buy this used Z400, I found many web pages of used PC shop or auction sites that advertise used Z400 that has 48GB memory in it. Why don't you try to search by google with 2 keywords of "z400" and "48gb"?
The next pages were found today, for example:
amazon Japan: http://www.amazon.co.jp/%E3%80%90%E4%B8%AD%E5%8F%A4%E3%80%91-6DIMM-XeonW3565-3-2GHz-MULTI-NVS295/dp/B01C8AVASA
CompuRAM: https://www.compuram.biz/memory,hp+(-compaq),desktop-1-workstation,hp+workstation,z400.htm (see Tech Specs note).
Anyway, I confirmed it works fine without problem until now after I installed Win7 Pro x64 (ja-JP) yesterday. (only 1-2 hours has passed though...) I will test further more from now on. I will post here if I encountered any issues.