Before you install OpsMgr 2012, be sure to work through the OpsMgr Pre-Upgrade tasks. Pay particular attention to the collation on your SQL server installation. If your collation is set to anything but SQL_Latin1_General_CP1_CI_AS, you may want to look at changing your configuration. You can find help on doing this here and here.
And of course, be sure to work through the installation prerequisites too. You will need to request and download the Authorisation Manager hotfix for both a clean installation and an upgrade from OpsMgr 2007 R2.