Service Packs:
We generally install them on one system, and if there appear to be no problems we go ahead. The bottom line is that MS is pretty good these days with the service pack releases for Windows 2000 OS. We dont worry about the 2000 service packs: besides if there is a problem you can always roll the configuration back to the prior one - option at setup.
Windows 2000 is a great OS and we have been running it since release with 99.9 percent uptime which is much better than the prior releases. In fact the entire 2000 server line: Exchange server, SQL Server 2000, and ISA 2000 are very very stable and reliable, and these days they are relatively low cost to maintain especially with the built in enterprise management layers and tools: much better than previous releases.