Do not buy a used helmet. The impact absorbing lining is designed for *one* impact, be it when you crash, or just when it slips off the handle bars. Also, the liner is only good for 3-5 years. By that point the lining has oxidized (I think it's oxidation) to the point where it will no longer be effective.
The top two brands are Arai and Shoei. Pricey.
The next tier is HJC and Scorpion. I ride with an HJC AC-12. There are a couple others at this price point, but I can't remember the brands at the moment.
At a third level you have Bell helmets. They'd probably protect you in a crash, but not as nice to live with day to day.
You definitely want a DOT rating, and I'd prefer a Snell cert as well. I was under the impression that these endorsements were indicated by the applicable sticker, so I'm going to disagree with Bedlamite on that point. I'm pretty sure that if it carries the sticker of an organization, it has been approved by that organization.
As far as styling and features, I like a full-face helmet. I feel that they offer superior crash protection, specifically to the face and lower jaw. They will also help to shield your face from wind, rain, bugs, and debris. Vents are a good thing to have, especially when your face shield fogs. I also appreciate quick release systems for the face shield, they make it much easier to swap between tinted, clear, etc.
A web resource that I like to recommend is one of the other forums I hang out on, totalmotorcycle.com. Lots of great people that are good about helping out new riders.