Microsoft released the RTM version of Windows XP SP2, which will be available through Windows update/Automatic updates service in the next few days. Because of the size (about 265 MB), you might also consider downloading the package through the TechNet XP2 Site or MSDN Subscriber Downloads.
I guess the most obvious changes you'll notice after installing SP2 will be the new Windows Security Center, improved Firewall and handy pop-up blocker. However, the SP2 focuses on stronger security and better manageability. Microsoft sees it as an essential security update, making this package a required install.