Elanja Johnson did a fabulous post on researching literary agents by utilizing QueryTracker on the QueryTracker blog today. There's also a bit about personalizing queries.
Something I'd like to reiterate is that you owe it to yourself (and it's a courtesy to the agents you're querying) to try to find a good match.
I realize that it's hard to find information on some agents. If you feel you must give these agents a shot, I'd recommend you know (at least) what genres they represent (yours?) and whether or not they have legitimate sales and/or a respectable seat in publishing. If you can't find even this information, why are they on your list? And if you're going to query an agent with this amount of information, do yourself a favor, have a prepared list of questions in case THE CALL does come.
I constantly see writers out there cold-firing queries, doing little-to-no research, and/or accepting the first offer they receive without question, and it just doesn't make sense to me. I can understand the desire to obtain representation and to get published but not the desire to get published no matter what. Personally, I want the best for myself, my books, and my expected career as an author. You should too. The goal is not to be a one-hit wonder, it's to build a career, and you're going to need strong, lasting relationships to make the most of it.
My closing advice: If you query all your great, good, and decent matches and still don't have representation, don't query everyone else under the publishing sun, write a new book or extensively rewrite.
Whether you agree or disagree, feel free to open up a friendly discussion in the comments.
And thank you, Elanja, for the great post!