This is an important milestone in preparation for Exchange 2010. You can grab it here: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=4c4bd2a3-5e50-42b0-8bbb-2cc9afe3216a

The service pack is so large because it is the complete installation files. You can install a new server using this download only. The Exchange 2007 SP2 Build number is 8.2.176.2

Go through the Exchange 2007 Service Pack 2 Prerequisites:

• Backup System State on the Schema master
• Patch the Exchange server OS to the highest level
• Extend the Schema
• Prepare Active Directory
• Install Windows Installer 4.5
• Uninstall Interim Updates

See  the full details at: http://msexchangeteam.com/archive/2009/08/28/452209.aspx

Exchange 2007 SP2 is able to install in a Server 2008 R2 domain but it wont be supported to install on Windows Server 2008 R2 machine. See this post on why: http://msexchangeteam.com/archive/2009/09/21/452567.aspx