get an SA2000 rated helmet.
i got an M2000, and regret it cuz i dind't know any better.
it's fine for autoX, but serious events laugh it off.
M's are designed for one big hit.
S's are designed for tons of little hits (ie, your head against the roll cage hopping along) as well as one big hit, and adds a level of fire protection to your dome.
If you wait a year, SA2005 will probably be out. generally, you have 10 years of life from the rating. M/SA95's are going out next year.