You've pushed cross-linking relationships. You're taking subscribers to an e-mail newsletter. Nothing could be finer.
Sales are the obvious indicator.
If you're selling enough to cover your costs, that's a start. If you're seeing incremental revenue growth from your numbers since you went online, all the better. But to really optimize your e-business, you need to really know your customer. You need to capture demographic information. You need to listen to complaints and kudos pouring in through your e-mail box.