Assume an attractive website is an effective website
I’m all for great design. Great design alone can help sell a product, but an attractive website that doesn’t appeal, navigate, or communicate the right message is not of much value.
Ultimately, great web design comes down to combining elements in ways that make it easy for people to engage with the content, learn and do pretty much what you want them to do without having to think about it.
While there are proven design conventions for the web, testing is really the only way to get it totally right. Usertesting.com is a great place to start. Get your customers to use your site while you watch and do plenty of A/B testing using a tool like Google Website Optimizer.