Microsoft have just released an 11 page document entitled Windows XP Service Pack 3 Overview.

It lists out some of the Windows technologies new to this version

including:

Black Hole Router Detection
Network Access Protection (NAP)
Descriptive Security Options User Interface
Enhanced Security for Administrator and Service policy entries
Microsoft Kernel Mode Cryptographic Module
Windows Product Activation

Here is a snipped about deployment.

Windows XP SP3 will be available through Windows Update and the Microsoft Download Center.

Through Windows Update, the download size varies, but it is typically 70 megabytes (MB), depending on the computer’s configuration. Through the Download Center, the download size is approximately 580 MB. Fundamentally, deploying Windows XP SP3 works the same as deploying SP1 and SP2 for Windows XP:

SP3 is cumulative, so users can install SP3 on top of Windows XP SP1 or SP2.

Windows XP SP3 supports the same languages as Windows XP did in its initial release.

You can run the SP3 update package on any SKU of Windows XP SP1 or SP2. For example, you can run the SP3 update package on a computer running the Windows XP Media Center Edition with SP1.

Link: Download on this page