There is a great post over at Problogger entitled, “How to Decide How Many Columns are Best for Your Blog

Darren recommends a ‘design by function’ approach. Before developing even a concept of what you want your layout to look like take a step back and think about the purpose of the blog. What elements do you want to include? Which areas are most important? What will make the experience the best and easiest for your visitors? Focus on what provides value to your visitor and design from there.