I'm a full-time web developer and the i-Net+ exam helped me understand the basics, but it didn't go too far beyond that. It's something to have under your belt, but don't expect it to take you up the ladder. I used the Exam Cram book as well - and it's basically all you need, especially if you already have experience.
In graphics design, you may want to look into the Prosoft Certified Internet Webmaster series (CIW)...
http://www.ciwcertified.com ... it doesn't yet hold much water though (I've been saying that for years, unfortunately).
Truth is, in this field, your portfolio is already your best asset. No need to prove what you know conceptually.