I second tipstir's comments. Generally I advocate updating, but I don't believe any significant upgrades (or any at all really) were put in with SP3. Instead it was really just a bunch of security updates that you can also get on SP2. At least that is how I understand it. SP1 got program access and defaults in the Add/Remove Programs control panel, SP2 got a Security Center, SP3 just got updates. SP3 is also the only one that you can't directly install on a pre service pack XP. So its hardly a service pack in the 'traditional' sense.