<--Warning! Focus newbie!
If you are going to be doing HPDE (High Performance Driving Event) on a road track then they will probably require a Snell rated SA helmet as they are made with fire resistant materials. If you are just doing autocross (my specialty) then any motorcycle helmet with a Snell M2005 sticker in it will do.
Important- Whatever helmet you buy it must have a Snell sticker inside or you won't be allowed to use it.