NOTE:  Release Candidate 2 of Service Pack 2 is still Beta software, and may cause unforeseen problems on your system.  This shouldn't be installed on a system which you cannot afford to reinstall. The final version should be with us Summer 2004.

In terms of Media Center this will update any previous non beta version of Windows XP Media Center 2002 or 2004 to build 5.1.2600.2149. This is mainly a bug fix/performance update rather than adding new features.

Windows XP Service Pack 2 Release Candidate 2 (Full Network Installer): 
http://download.microsoft.com/download/C/1/6/C169BA60-E2D6-4C5F-B4F2-23DA024C9FA8/xpSP2.exe

Service Pack 2 RC2 Release Notes:
http://www.microsoft.com/technet/prodtechnol/winxppro/deploy/relsprc2.mspx