XP SP1, do I need all the SPs?

Sorry if I'm duplicating. I thought there was a similar thread to this, but the search isn't finding anything. I'm redoing a PC, so it now has XP Pro SP1. Do I need all the Service Pack updates, or just one? Which would you recommend I keep? Is it best to get the highest # or stick with a good one? It's only a Pentium III 500Mhz with 1/4 Gig RAM (256MB), although I may add more in the future. Also, where do I download them, since only SP2 is listed in Windows Update?

 

Technically, you don't 'need' them as it will still work without them but it is a very good idea to keep up to date (SP3) for security and reliability. Be sure you have the latest drivers before you start updating, though. Microsoft recommends that you use Windows Update to install service packs and updates. This is a link for downloading the SP3 package if you still want to go this route:
SP3 download

 

Use Just Playins advice, but my understanding is that it's best to install SP2 before installing SP3. SP3 pukes out sometimes if SP2 is not installed first.