Use Windows 2008R2 (which is 64 bit only) for sure. 2008 acts to much like Vista.
The wsisa.dll performance is excellent. I have found that we could consolidate 4 to 8 web servers (Win 2003 32bit using cgiip or wsisa) down to a single server.
Between the improvements in Windows OS, IIS 7.5 and Progress 9 to OpenEdge 10 its a night and day difference.
Just a few extra things you need to do so follow KB: P125146 & P154840. Change the Application Pool Maximum Worker Processes from 1 to 4 (this is for better performance), Change Rapid Fail Protection - Enabled from True to FALSE.
Use the Output Caching for any of your static file extensions.
Interested in hearing other performance improvements others have found.
Lynn