Most server processes which run as a Windows service can run as within a cluster.
Take a look at SQL Server Magazine’s December 2008 issue, in it I cover exactly how to build a cluster.
As for the requirements, for Windows 2003 the hardware must be on the Windows Cluster HCL on Microsoft’s web site. For Windows 2008 the hardware must be able to run Windows 2008 and pass the cluster test process.
In either case you’ll need some sort of shared storage like a SAN so that both servers can access the same hard drives.