Does the size and layout of product grids on an eCommerce site make a difference in usability and sales? Hugo Jenkins attempts to answer this with case studies involving different column and product thumbnail sizes. He found some clear losers (five column layouts) but no clear winners.
Sharing test results like this should encourage other eCommerce site owners to perform similar tests and compare their results. But beware of how you setup the test; his test was quite small, which was okay for a usability test where you track several elements, like time on the site; but I wouldn't track just sales volume differences with such a tiny sample.