You can get all kinds of helmets, my personal preference was for the SA2000 rating. My helmet only cost about $250 and I got one with mouth protection.
"White, large, Integral GT helmet
With a lightweight Compo-Tech fiber shell and a flow through aerodynamic visor with sun strip, Bieffe's Integral GT helmet is a popular choice among World Touring and Rally Championship drivers. The Integral GT features a removable, washable Snell 2000 Nomex comfort liner for safety and a comfortable fit, plus an attractive white outer shell with superior styling."
While it's not the most expensive helmet available it's decent, practical.. doesn't look like a motorcycle helmet. I picked mine up from Summit Racing.
you can go here to check on certified helmets
http://www.smf.org/cert.html Remember SCCA uses the SA rating they may accept other ones such as the M, but the SA is the standard for auto-racing.
Good luck, I did about 3 months research last summer before I picked mine up. For autocrossing and the occasional track foray, I don't think you need to worry about purchasing a 600 to 2000 dollar helmet. They are safer somewhat, but I can't see the justification.
