Do visual studio 2010 include already Sql Server instance, or I need to install Sql Server developer edition to develop an application that need a Sql Server db.
If it installs a Sql Server express edition, it this enough or it's better to have Sql Server developer edition?
The Visual Studio 2010 installer gives you the option of installing SQL Server 2008 Express Edition. This edition of SQL Server is good enough for development purposes but is not intended for production use.
I would second that installing the "full" SQL Server Development Edition makes sense if you do serious database development.
However you can have both installed side by side, and having SQL Server Express is very useful for one reason - it supports placing database files in the App_Data folder of ASP.NET applications. While I don't do this for my own projects, it's a very nice feature for demos or open source applications you download from the web: just unzip, start VS, hit F5, and you have a running solution including database. Without SQL Express, you first have to move the MDF file, attach the database, set up user rights, replace the connection strings etc...
I think the size limitation is not the main issue. In Sql Server 2008 you have some features in Sql Server Management Studio that aren't available in the Express management Studio or the Express database engine, like: - suggesting the table and column names (in Query Window) - Sql profiler Cor Westra