Having a web presence for an artist can take many forms, depending on what you want out of it.
1. Starting a store is easy. But... There are no shortcuts to making a web presence that gets a lot of traffic and makes a lot of money. People think that if they open a store on Etsy, or Cafepress, or wherever, that money will start falling from the skies on them. Definitely not the case. I have tried that approach. The principle of getting oneself out there and working the crowd has not changed since the old days of gallery openings with free food. To gain attention of large numbers, you will need to constantly promote a presence to raise yourself above the ambient noise of the web (unless you have made the next dancing hamsters, but the likelihood of that is on the order of winning the lottery). So pass on the self-delusion, and recognize that like anything worthwhile, it takes work.
1.b It's about link-backs, or is it? For the great unwashed masses to become aware of your store, they will need to find you when they search. Making a store that gets moved up the search results does require link-backs. That is where the work starts. Link-backs come from you convincing someone elses web presence to place a link to you on their site, usually on the condition of you reciprocating with a link on your site back to them. This used to be called "networking" and it takes time and effort. Now here's the caveat... If you don't care about large numbers, then the above discussion doesn't matter! If you have a very targeted message, that will be beamed directly into people's email boxes, with the link-back: "Hey, I just opened up my store come visit". This will work for friends, curators, or other professionals that you can directly target, probably not enough to keep you in champagne and caviar, but that may not be the purpose. "Keeping up with old friends vs. making new friends" is what it comes down to.
1.c Your content is like a salad, keep it fresh. The next unrecognized time commitment for a store is that it needs to be tended. To get people, even friends, to come back, your store must convey the expectation that next time will be different from last time, and it will be getting better and better. By this I mean that you have a responsibility to maintain the quality, and getting gradually better (what is called "artistic growth"). Set yourself a realistic schedule for updating your content, something you can sustain for the initial phase of your web presence.
2. These are all true for starting a blog as well. There is so much noise out there that rising above it is your biggest hurdle. Having really good content is the best place to start. As an artist that is what you do! You are what is called in the business a "content generator". The web is starved for good content. Emphasis on "good". Once again, networking your blog and keeping it fresh are the time consuming parts of the process.
3. These are not all true for a gallery site. People visit museums where the content was created hundreds of years ago, so your web gallery should be okay if it isn't kept as fresh as your blog or store. How often do you update your CV? How often do you complete a new body of work? These are questions that can be answered at your leisure, though you will want to avoid the impression that the gallery is orphaned (no longer being updated).
I have been very happy with BestThinking as a platform for my online gallery and my blog. No store I have ever created has paid out big for me (lulu.com, cafepress.com, etsy.com), but then I have not followed my own advice in a, b and c above.
I hope this answers some of your questions - let me know what else I can tell you about.