It's an extensive read but might come in handy to know exactly what all the SP contains.
"This article lists the problems that are fixed in Microsoft Windows XP Service Pack 2 (SP2). Service packs are cumulative. This means that the problems that are fixed in a service pack are also fixed in later service packs. For example, Windows XP SP2 contains all the fixes that are included in Windows XP Service Pack 1 (SP1). You do not have to install an earlier service pack before you install Windows XP SP2."
http://support.microsoft.com/default.aspx?scid=kb;en-us;811113
"This article lists the problems that are fixed in Microsoft Windows XP Service Pack 2 (SP2). Service packs are cumulative. This means that the problems that are fixed in a service pack are also fixed in later service packs. For example, Windows XP SP2 contains all the fixes that are included in Windows XP Service Pack 1 (SP1). You do not have to install an earlier service pack before you install Windows XP SP2."
http://support.microsoft.com/default.aspx?scid=kb;en-us;811113