In order to build a Windows Cluster (which is what is needed for a SQL Cluster) you'll need two servers (usually of the same spec) and shared storage. If you are going to cluster Windows 2008 you'll need either a SAN, or a SCSI device that supports SCSI 3, as SCSI 2 is no longer supported for shared storage. I'd recommend a fibre channel SAN as they throughput is much faster.
Pretty much the only other requirement is that the servers will need to have at least 2 NICs each which is pretty standard these days.
Setting up a cluster if you don't already have a SAN isn't cheep. The storage array alone can quickly get up into the 6 figure price range rather quickly depending on the vendor and the options you get.
When it comes to storage keep in mind that you pretty much get what you pay for. Check out the December 2007 issue of SQL Server magazine. It's got an article on clustering SQL Server (granted it's for SQL 2005). There will be an updated version of the article coming out in the September (I think) issue of SQL Server magazine which focuses on clustering SQL Server 2008. You are also welcome to join me on September 15, 2009 when I give a webcast about setting up a Cluster over iSCSI (same basic concepts as fibre channel, just using Ethernet instead of FC).