Once you move into the Windows Datacenter Edition world the hardware is highly available even with only one server.

http://www-03.ibm.com/systems/x/solutions/os/windows/datacenter.html

http://www.microsoft.com/windowsserver2003/datacenter/dcprogram.mspx

http://technet.microsoft.com/en-us/windowsserver/bb429508.aspx

and you have support levels from the vendors that are way beyond anything you get with a commodity server.