And other things? If you are trying to run a bunch of open source stuff then perhaps you are going to run into compatibility issues - I don't have much experience trying to do this on windows. Presumably with IIS 7 and it's built in fcgi support PHP should be doable. If you are looking at hosting any of Microsoft's server products like exchange, sql server, SharePoint, or basically any new server they come up with, the 64 bit version of the OS is recommended if not required (for instance with exchange 2007+ you can't use a 32 bit OS) Even the .NET stack for web I would go with a 64 bit OS. Server 2008 R2 is a really awesome server if you are in windows land.
EDIT:
How to enable fastcgi and fix some issues running PHP.
The fix from MS for windows 2008 64bit: