Dell.com design case study
Frog design has a nice case study of their work with Dell (albeit in a badly framed page). It covers some of the design rationale used to make specific design decisions like use of color to highlight main path navigation:

"We established a primary purchase path defined by bright, bold colorful buttons which lead the user down the path. To address other types of shoppers, less prominent monotone image links are used. To lead users through shopping paths, we established a system of way-finding link icons with primary ones leading to the next step in the path and secondary icons leading to tangent content."