Interested in all the new things in SP2? Here is a complete list.
And this is the complete download site with links for the SP2 itself, a new BOL, new Samples, new Feature Pack and especially the links to SQL Server 2005 Reporting Services Add-in for Microsoft SharePoint Technologies and SQL Server 2005 Data Mining Add-ins for Microsoft Office 2007.
To avoid a restart after the installation of the SP2 you should stop the neccessary services which could be done with the following commands on the command line - if you have used the standard instances:
net stop MSSQLSERVER
net stop MSSQLServerOLAPService
net stop msftesql
net stop MsDtsServer
net stop ReportServer
After the installation the "SQL Server 2005 User Provisioning Tool is started automatically if you use Windows Vista. As you probably know, Windows Admins are not automatically also SQL or AS Admins - this tool enables to set the right privileges to the administartor accounts.
But do not forget to start the services before you try to set the privileges...
If you want to add a new account you will have to start the Provisioning Tool again, but unluckily it is not added to the Start Menu, you will find it at:
C:\Program Files\Microsoft SQL Server\90\Shared\SqlProv.exe - assumed that you have a standard installation and an english version of MS Windows Vista.