To perform an upgrade, you must have installed Office SharePoint Server 2007 with Service Pack 2 (SP2).
Inplace Migration and database migration
Need MOSS SP2 for migration. Pre Upgrade Checker to generate reports.
MOSS has 15,000 site collection limition for single DB whereas 2010 has 5,000 limition. So devide site collection into multiple db before upgrade.
We do have option to keep visual as 2007 for 2010.