The best way to deploy System Center Operations Manager is to have a dedicated SQL server, or at minimum a SQL utility server that is sufficiently powered to provide a good experience to the SCOM functions. If you under-power SCOM, especially at the SQL side, you are going to have a poor usage experience with the tool.
If you've accidentally deployed System Center Operations Manager to the default SQL Express instance and you're not sure what to do, have no fear! The key procedural steps (from TechNet) are: http://technet.microsoft.com/en-us/library/cc540384.aspx
This worked well for me, however, I'd avoid deleting the old database (step 3) until you've actually gotten Operations Manager functioning on the other server. Instead, I disabled the SQL services on the old server.