Building a Linux Cluster from the Ground Up?

dooling asks: "How would one go about building a Linux cluster from the ground up? I read a lot about Linux clusters on /. and have been able to find some information on configuring the cluster, but have found little on how to assemble the hardware, i.e., what is necessary, how they should be connected, etc. So does anyone have reliable information on hardware assembly and configuration? Also, (if you've never done this before) is it worth building your own, or is it better to just buy one prebuilt and preconfigured? If you want specifics: 20-40 machines, Linux (probably RedHat 6.x), disk or diskless?, do not need video cards (but should we have them?), switch or hub (best way to hook them up). We will be doing pretty straightforward scientific computing (floating point number crunching). "