Hi,
All good advise so far. But one thing to be considered, mentioned in the previous Helmet thread but not yet mentioned here, is weight.
This especially true for Auto-X where your head is thrown around much more than most other track events. A lighter Helmet can really reduce the Fatigue Factor and be the difference between a sore, stiff, neck the next day, and not. It may even help your ETs because you're not fighting as hard to keep your head upright.
Most Helmets are made from FibreGlass, but a number are also now using lightweight Kevlar. In addition to be extremely light (compared to a FibreGlass one), they are also exceptionally strong (maybe even bulletproof - I haven't checked that one yet -

).
Some of the lightest Helmets now produced (and Snell 2005 certified) are from the French manufacturer Stand 21 -
http://www.subesports.com/products/cat/Helmets/brand/Stand+21/prodID/181 . They're spendy, but a worthwhile investment if you do a lot of motorsports. Just a few ounces less can make all the difference in the world...
Happy Motoring!... Jim'99