Is "Denali" is SQL SERVER 2011
Maybe, maybe not. Maybe it is SQL Server 2012, or MS decides to change naming. It IS, though, asi t sas clearly on the side, the next generation of SQL Server.
I mean is it a stand alone server or
just a feature addition to SQL Server
2008?
MS always has code names for in development technology, and if you read the side it is clear it is next generation SQL Server.
Than do I need to uninstall SQL Server
2005 [which is present in my system]
before installing "Denali" ?
Given that since 2000 or 2005 SQL Server supports multiple installs in different instances side by side- no, you do not need. tHat said, if you isntall Denali and your syste mbreaks you join the line of people realizing that MS is serious about "do not install beta technology on productive systems":
You are better off installing SQL 2008 R2.