what sort of cluster are you talking about???
beowulf cluster? (lots of nodes each doing part of a big task)
distributed system? (same sort of thing but longer distances involved)
plan9/grid system? (this is where you use resources like file pipes, so server 1 can connect to, and use the memory directly on server 2) -whilst this is a really awesome sounding thing, (and is completely free) plan 9 is a really old OS, made to replace Unix, it's more complicated than Unix, never truly replaced UNIX (because Unix is good enough) barely anybody in the world uses it.
SQL server cluster (a database cluster, -are you thinking you want active passive nodes, -2 nodes with the same data where if 1 server fails the other picks up and takes over, will the servers be in the same location or geographically separated?, or perhaps active active? where both nodes work at the same time),
Oracle cluster using Linux (otherwise known are Real application cluster) -where you have the same choices as the SQL server cluster, but you're using linux (don't be fooled into thinking that means free, Oracle is NOT free!
an exchange cluster (Microsoft's mail server -lots of clustering goodness there with the new hubCAS servers)
a clustered web server to balance traffic between multiple nodes??? (technically that'd be load balanced rather than clustered, but in the English sense of the work you'd have a cluster of servers working together.
or when you say cluster, are you really thinking cloud, where the servers run something link VMWare and virtual machines can exist on either server at any time, and you can literally pull out the network cables and machines stay on powered up on the other machine
lots of different options...
Is this for a practical work task (You need to create an exchange cluster).
or is this a home idea? you're bored and have a few spare machines?