As far as connecting to SQL Server 2012 any of the ODBC versions will work (6.0, 10.0, 11.0). The lower versions may not be able to handle more recent functionality/features of SQL Servers.
6.0, 10.0,11.0 refer to the following dll files:
6 : file name sqlsrv32.dll
10: file name sqlncli10.dll
11: file name sqlncli11.dll
There is an old article (sqlserver2005) that you may want to read:
http://www.aspfree.com/c/a/database/using-ado-with-the-sql-native-client/#more-532
This next one is about version 11.0