Check with the organization you're going to be running with (and any that you MIGHT run with--you're going to get hooked!) as to whether they require SA-rated (auto racing) helmets or if M-rated (motorcycle) helmets are acceptable. Most organizations do allow M-rated helmets for track days. (SA helmets are constructed with fire-resistant materials whereas M-rateds are not, and SAs also meet different impact criteria--rollbars, multiple impacts, etc.) Further, the ratings are updated every five years and many track organizations allow only the current and one previous spec, so you want a helmet with a 2010 Snell rating to get the longest life from it. You do not want a 2000-spec helmet, no matter how good the deal. Best bet for maximum eligibility/life across multiple organizations is a SA-2010 helmet.
Fit is critical. Here's a good guide (make sure to scroll down past the size tables):
http://www.racinghelmetguide.com/aut...et-sizing.html. Ideally, find somewhere local you can go to try on multiple helmets. Different brands fit differently... I went through two brands in two sizes each beofre finding the right one, and spent a lot on shipping sending them back.