We've been running on Win2000 SP3 for the past year. We've had some problems, especially the the Event ID 1000 Userev where the profile fails to unload. Win2000 SP4 is a nightmare for Citrix and terminal services....stay away for now if possible. From what I've read from other forums, the safest setup is running Win2000 SP2. If you're not having any problems that are SP related, I'd stick with what you have.