The file is 336.1 MB and contains all the previously released updates for Win XP.
The Service Pack is said to have a few new functionalities and according to the testing done by researchers the pack is said to make the OS upto 10% faster.
It is recommended to backup inmortant information and set a restore point before installing this pack. The support site also mentions that the SP3 can be easily uninstalled with the following steps.
Uninstall information for the Windows XP SP3 Release Candidate
To uninstall Windows XP SP3, follow these steps:
1. Click Start, click Run, type appwiz.cpl, and then click OK.
2. Click Change or Remove Programs.
3. Click Windows XP Service Pack 3 (936929), and then click Remove.
4. Restart the computer.
The Service Pack 3 is available for download here.
I am still downloading the file, will post more info when I get back.