I can't believe how important storage has become. I am working on some Hosted Exchange projects, and disk makes or breaks the environment. Dave Lalor from our internal IT group wrote an awesome paper with HP about their EVA storage. Unfortunately, it doesn’t apply to EMC or Hitachi. Matt Gossage also has some good stuff on performance testing with JetStress to look at I/O limitations.
With more people using Virtual Server for development and test environments, I highly recommend putting the images on high speed disk (the more small spindles the better). Disk is almost always the bottleneck hyper-threaded processors and memory can usually handle a number of images
BTW, I see Hitachi more and more and less EMC.