I was monitoring some community sources today and saw value in dedicating a post to guidance for both hardware selection as well as sizing for OpsMgr.
There was a post made in April last year. See below:
http://blogs.technet.com/momteam/archive/2008/04/10/opsmgr-2007-hardware-guidance-what-hardware-do-i-buy.aspx
Included was a hardware sizing suggestion sheet. I have attached the sheet at the bottom of this post.
Additional information is also available on sizing but it requires that you have an idea of the number of events that will be gathered. See the product documentation guides for further details on this.
http://technet.microsoft.com/en-us/opsmgr/bb498235.aspx
Strictly speaking, the OpsMgr server itself will depend on whether you are using a distributed application installation with the various roles spread on different servers, or an all in one solution where the SQL 2005 database(s) will be housed on the same server that the RMS server role is installed on.
Reporting and data retention periods for performance data will also impact your choices as will the decision of whether you will use audit collection services (ACS).
For great, in-depth, technical training on System Center, check out www.infrontconsulting.com/events.htm.