Visual Studio 2005 Service Pack 1 Install Requires Big Resource
Yes, in terms of manhour, computer time, plus hard drive space.
Standard installation of VS2005SP1 is 3 hours, with many people experience longer. That itself accounts to half day work.
Another resource taken is hard drive space. Standard installation requires at least 3Gb free (while installing, afterwards it's cleaned up). If you have multiple Visual Studio 2005 installation, it would need up to 6.2Gb free to install successfully.
Don't think about failing though, the best case would be just reinstalling the SP1. The worst, well, total Windows clean up (and this would take another half day work).
You can take the shortcut way of doing it once and deploy it anywhere using a method called slipstreaming. Heath Stewart has published the way to slipstream your Visual Studio 2005 Service Pack 1 here.