I've always subscribed to 'content is king' - but a poor site design can still hamper sales no matter how dazzling the content.

There'll always be a percentage of people who judge a book by its cover, regardless of how good or appealing the actual product is inside. If the tour sucks, the graphics are bad and the site is confusing to navigate - chances are many will assume the content is similiarly inferior.

Trial and error is the best way of approaching it. Try your best to ellicit feedback from visitors on their overall opinion - both good and bad. Offer some sort of token reward for their input (perhaps a 24hr. limited trial membership to a specified area within the pay section).