Check out the following sites and their associated links:
"Why choose iSeries as your Domino server?"
http://www-1.ibm.com/servers/eserver/iseries/domino/value.htm
"A TCO Analysis of IBM AS/400 Dedicated Server for Domino versus PC Servers"
http://www-1.ibm.com/servers/eserver/iseries/conslt/dsd_tco.htm
Also note that in the above report it states that "Survey-based research indicates that 500 users are the approximate limit for an individual PC server."
In your decision you need to also consider scalability.