It is hard to answer this question. It depends a lot on the way your domino server is used : mail only, mail and applications ? Web access is enabled for mail, applications, both, none ? Do you use DOLS, DECS, … ?
I have no experience with IBM x-series, but I have several domino server (6.5.4) running above W2k on Dell machine.
I run easily (load below 20%) 600 users on a similar config for mail and mail archiving primarily plus a few little applications.
On the other hand, the application servers (plus a few mail enabled applications) that is the same config but with more than 100 applications and thousands scheduled agent saw its performance decrease drastically after having passed the cap of ~400 users.
Both server are accessed indifferently through NRPC, HTTP, HTTPs, POP3 and SMTP.
The application server is also a LDAP server.