Hello d3r,
Service Pack 3 is primarily just a collection of updates and fixes released for Service Pack 2. There has been a couple of small enhancements, but nothing like there was for Service Pack 2 with it's Security Centre. Hopefully this page should help you with any questions you might have:
http://www.winsupersite.com/faq/xp_sp3.asp
Many have updated their OS without any problems, but there has been some complaints that SP3 has caused problems. I would say that unless you are interested in Service Pack 3's new features, and you already have all the latest updates and fixes, then there is no need to install SP3.