Most race circuits will demand you use a helmet that's rated to the most current Snell Foundation standard...but they will frequently accept a rating that's one standard older.
For example, right now the Snell SA-2000 rating is the standard, but you can get away with an SA-95. However, the new Snell rating is expected in 2005, so the SA-95's will no longer be allowed.
Also, it's important to use an SA ("special applications") helmet rather than an M ("motorcycle") model. The SA helmets are tested to withstand multiple sharp impacts, like what happens when you roll a car and your head is rattling around inside a cage. In contrast, M rated helmets are only tested to withstand a single, large impact, like what happens when you fall off your bike and smash your head into the curb.
Emre