All 'serious' helmets will have a Snell rating, along with the year of the Snell standards the helmet meets. Current skid lids are Snell 2000, which will shortly be superceded by Snell 2005. Standards are upgraded every 5 years.
Most motorsports endeavours will require a Snell 95 at least. In addition, there are the M and SA designations. (e.g. Snell 2000SA). The M designation is for motorcycle helmets, the SA is for motorsports helmets. The SA is similar to the M but with the addition of fireproofing and the ability to withstand impacts with a roll cage. An SA helmet is fine for motorcyling, while an M helmet is not fine for motorsport.
Most motorsport will require a full face helmet, especially in a street car with normal seat belts. A violent impact can stretch the belt (or you may be wrenched out of position) enough to impact your chin on the steering wheel. This is un-good, but a full face helmet will provide a measure of protection.
If you are going out on the track, even just for Triumphest type events, you should have at least a Snell 95SA full face helmet. How much is your brain worth to you?