From my experience as an web consultant, I can tell you that page design is critical. Online retailers often test multiple variations of page templates to measure effectiveness. In doing so, there's considerable strain placed on the web design team as they produce fully colored and refined Photoshop mockups.
One solution I've found to be an effective tool in the e-commerce web design arsenal is a wireframe diagram. While I wouldn't necessarily recommend this approach for other types of web sites, wireframe diagrams help designers take a rapid-fire approach to e-commerce page design.
Larry Becker at the Rimm-Kaufman Group recently published an excellent article on MultiChannelMerchant.com that discusses how getting back to pen and paper (or simple digital line drawings) can yield great results.
I've found the lo-fi prototype to be an effective tool when doing site assessments to show e-commerce clients how their pages should be laid out for maximum effectiveness. Wireframes take the subjectivity out of the design and make the client focus on page layout, not color and image choices. While color and images are certainly important, those are considerations for after a page layout has been determined.