If you are thinking about Sprints or any other MSA event even as an off chance then it makes sense to buy and MSA approved helmet which will also be Fire Retardent. Snell SA2000 goes out very soon, so get an SA2005 or better a SA2010. See here http://www.msauk.org/uploadedfiles/msa_forms/bluebooks/13/151_189_Common_Regulations_for_Competitors_(Safety)_(K).pdf Page 166
Whilst BCF's link is interesting if you are riding a motorcycle, Motorsport car helmets have to meet different criteria which motorcycle helmets rarely meet. Fire Retardency is the most obvious, but also the type of impact is generally different between a car and a bike. A bike normally getting multiple hits on the road, compared with a car helmet, getting one big hard hit, followed by other smaller ones. The Snell Institute looks at Car racing helmets, so my personal preference would always be to go for a Snell Approved helmet. But as with all things it's a matter of personal choice.