Karen,
You don't want or need CI - it's old technology (almost 30 years!) requires special adapters (expensive), extra infrastructure (a "star coupler") and doesn't make any sense without CI based storage, which I'm sure you don't have (it's old, slow, low capacity and very power hungry by today's standards). These days CI has no advantages over network interconects and many disadvantages. [don't get me wrong, in it's day, CI was revolutionary, and by far the best solution for shared storage, but times have changed, and CI is now past it's "use by" date]
Your first cut at clustering can use your "house" network. Unless you're doing a lot of MSCP serving, the network overheads for SCS are likely to be little more than noise on a 100Mbit network. With modern switches, there should be no impact on other network traffic outside the cluster.
If there are spare network adapters you can (and, IMHO, should) simply plug them in, either connecting 2 systems directly by crossover cable, or connecting multiple systems to a switch or hub (nothing fancy, a cheap commodity switch will probably be more tha adequate). No configuration is necessary, the cluster will automatically find and use any available paths between cluster nodes.
Extra paths improve cluster availabilty in the event of network failure, and can improve throughput. For most purposes, just plug in the cables and forget about them, but if you have unusually high cluster or network traffic you can use SCACP to prioritise traffic on particular links. My advice would be to wait until you can prove there's a traffic issue before worrying about try to "fix" it ;-)
A crucible of informative mistakes
