First off, definitely use a cover letter. Don't use your template, write something specifically about why you want this job at this company. Be sincere.

With regards to a suit; here's what I've always done. I suppose it's a little risky, but it's always been fantastic for me.

Call the company you're interviewing, at whatever public number you find. Hopefully you get a receptionist, or at least someone gabby. Tell them that you're coming in for an interview, and you'd really like to dress appropriately - so instead of asking for advice, ask what their boss is wearing that day. If the answer is slacks and a button-up shirt (short-sleeved or not), then you're going to wear a coat and tie for the interview. If the answer is jeans, then you're going to wear slacks, polished shoes and a button up shirt.

You don't have to be shy about this, companies really like to know that you give a shit about what they think. It definitely shows when I'm interviewing with someone who knows why they want to work with us versus someone who just wants a paycheck.