We are in the process of upgrading from 6.5 to XIR2. We are getting a new web server and app server but then I was asked if we need a new CMS server.
BO4u789 (BOB member since 2006-12-01)
No, the CMS doesn’t necessarily need its own server. There should be info on the Business Objects support site about sizing, planning your deployment, etc. Information is stored in the repository but the structure in XI is completely different from 6 and previous versions. Reports and universes are not stored in the repository, only the information that enables Business Objects to find them in the file system.
If you’re responsible for the upgrade and are going to be looking after the XI system, it’s worth getting some training. Business Objects offer online courses as well as classroom courses - I don’t know where you are located but it’s definitely worth finding out more about XI before you go much further. There is a 3-day migration course as well as the XI admin courses.
Gillian 
(BOB member since 2005-08-18)
CMS is a service that talks to the system database. Many people refers it as the system database itself.
If it is the database that you are talking about, it needs to be in a separate server. You should never install the database on the same server as the application. Competing processing resources is one thing, maintenance conflict is another.
substring 
(BOB member since 2004-01-16)
Here is some useful information on the XI R2 Architecture to get you started
EnterpriseArchitect (BOB member since 2006-03-23)