THE HOLY GRAIL: Considering whether to add Internet advertising to your marketing plan's media mix? You're not alone. Many companies we talk to say they aren't satisfied with the current methods used to track the effectiveness of Web-based advertising. Click-through rates just aren't enough for them. What many of them say they want is a system to track the first time a potential customer clicks on a banner ad all the way to eventual sale. Agencies know this is what many clients are waiting for, but so far no one has figured out how to do it.