Just a quick note - if you're planning on upgrading Windows Server 2003 to Service Pack 1, and you have ISA 2004 Standard Edition already installed, you should install ISA 2004 SE Service Pack 1 first - it copes with a few of the changes in WS2003SP1 better than the original release.

ISA 2004 Enterprise Edition is equivalent to SE SP1, so shouldn't need fiddling.