Future Now out of NYC, who I know through Bryan Eisenberg who is our client with The Web Analytics Association, has a great newsletter on conversion rates. You can sign up on their site at http://www.grokdotcom.com/ and the latest conversion rate newsletter was just released.
You may also want to check out the Eisenberg’s book A Call to Action which is also about web marketing conversion rates.
My main point is that driving traffice to your site through PPC or regular search engine marketing doesn’t add a ton of value if you don’t CONVERT that traffic. While at a high level the web marketing fundamentals article definitely stands, at an incremental improvement level you can’t beat the detailed analysis of a conversion rate specialist (or become one!).