What's the effect of checking "Configure Enterprise Manager" on the step 4 of the database creating using dbca ?
It will attempt to create a dbcontrol configuration. dbcontrol is the single-database version of Enterprise Manager, not to be confused with Grid Control.
I say "attempt" to create, because experience has shown that if you take this option at the time of database creation, it is likely to fail because the dbcontrol needs to connect via the listener, and that won't be ready at the time dbca tries to configure dbcontrol. Don't ask me why because I don't know. But my experience, and that of a lot of people on this board trying to figure out the problem, has been such. As I result, I never check it but instead create the database, then come back with dbca and select to simply configure dbcontrol on the previously created db.
Which databases objects are created ? windows services ?
Objects - the em repository, owned by SYSMAN.
Windows services? I avoid Windows every chance I get, but I believe there will be a service to support the dbcontrol agent and/or console. Your own testing could quickly provide the answer.
I'm using OEM CC 12 Cloud Control to monitor my databases is it useful to check that option ?
Redundant at best. Maybe even colliding with each other. I don't know because since dbcontrol and Grid/Cloud control are redundant I've never tried to run both on the same database.