Welcome to Wmw, Jake.
To be perfectly honest, I've pretty much stuck with plain vanilla HTML, trying to keep code as search engine friendly as possible, as well as easy to maintain. Therefore, I'm sure others will give you much more sophisticated advice than I can.
DHTML is wonderful, but does have compatibility issues, but it fine as long as it properly degrades, as mivox pointed out.
>trying to step into the realm of professional site design
Once you step into this realm, with the accompanying site updates that occur, as well as the amount of repetitious coding that has to be done on larger sites, what I would suggest for immediate study, sooner rather than later (like me), are two things that you'll find to be tremendous time-savers:
1. CSS to avoid repetitious page and text formatting. As mivox mentioned, it needs to degrade gracefully, and although there are some who choose not to use it for one reason or another, there is one person of my acquaintance, for whose work I have the greatest admiration and respect, who I consider to be one of the most capable, highly talented web designers around, who always uses CSS. Not only are his design skills superb, but he is a dynamite SEO. Based on seeing his incredible work and learning as much as I can from him, I will be incorporating CSS into my sites.
2. Learn to use Server Side Includes for site navigation, a tremendous time-saver, and better to incorporate sooner rather than later. There are also other uses, but avoiding the grueling task of changing pages and pages of navigation code was my initial motivation. I would never use hosting that wasn't equipped for it, unless it was a site tha would never need updating.
I say SSI without hesitation because right now I'm in the middle of a site update, adding directories and pages. Not only does all the navigation on all the pages need to be modified, but in going through the stats in detail, I now know that certain keyword phases are pulling more traffic than others, and know that there should be individual pages added to get specific rankings for those particular products. This was enought to convince me to switch over to using SSI with any site that will need pages and/or products added, at least on the first update if not to begin with.
Another basic is to learn to use a bit of CGI. Knowing how to use Formmail (or the equivalent)for forms is critical once something is needed beyond the simple mailto: email link. Formmail is almost universally used, so common that I consider it the best place to start, since it's installed by default with so many web hosts.
Last, I'd recommend a big coffee pot and a big bottle of vitamins so you don't need much sleep. Read and learn as much about SEO as you can. I may be biased, but I believe that a search-engine-friendly skill-set for web design is foundational. Much better to do it right in the first place than to have a gorgeous site that cannot get rankings and have to tear it apart and redo it.
Another reason to learn SEO, aside from the fact that it will give you an additional, invaluable service to offer, is that knowledge of it helps immensely in acquiring new clients. For example, there was one particular site I had (since taken over by the former client, who redid it themselves), that got me a steady stream of serious inquiries. Like one local lady said when she called on the phone, "I typed in my product at the search engine, and the site came up. It was just THERE!" That site, although very simple, had one or two people a month wanting one like it for some reason (note: that didn't mean they did it), but that would not have happened if they hadn't found it in the first place.
Another thing you'll have to deal with is doing ecommerce sites, so becoming familair with the different shopping carts and payment options that are available is also an important skill. A related, important issue is dealing with dynamically generated pages, if you'll be working with those at all.
>point me in the direction of a specific resource
You've found it ;)