It looks like XP Service Pack 3 has been released to manufacturing and will be available for download April 29th.  I don't think there is much new in Sp3 other than a rollup of fixes since Service Pack 2 was launched. If your rebuilding an XP system it will be a lot simpler installing SP3 than downloading the hundreds of hot fixes and patches

Today we are happy to announce that Windows XP Service Pack 3 (SP3) has released to manufacturing (RTM). Windows XP SP3 bits are now working their way through our manufacturing channels to be available to OEM and Enterprise customers.

We are also in the final stages of preparing for release to the web (i.e. you!) on April 29th, via Windows Update and the Microsoft Download Center. Online documentation for Windows XP SP3, such as Microsoft Knowledge Base articles and the Microsoft TechNet Windows XP TechCenter, will be updated then. For customers who use Windows XP at home, Windows XP SP3 Automatic Update distribution for users at home will begin in early summer. 

Windows XP Service Pack 3 Released to Manufacturing - TechNet Forums