www.cs.virginia.edu/stream: The STREAM Benchmark provides a simple measure of the performance of the memory system on a node. This site also includes results for a wide variety of platforms, from PC nodes suitable for a Beowulf, to workstations, to supercomputers.
www.tomshardware.com: Aimed at hobbyists building their own computers, this is a good site for general background on node hardware and includes up-to-date instructions on building your own node.