Spend the $ on a good helmet. Also note the SA2005's are out this year. I just replaced my SA95. Unfortunatly helmets do need to be tried on. Bell is the only one that fits my dome, Arai, simpson, impact to name a few, just do not fit me they put a lot of pressure on my forehead.
The biggest difference in helmets, besides quality of appointments (liners etc), is weight. Assuming you are driving hard for the day, the difference in a few ounces you will feel the next day (lighter the better). Also if you are going to be getting serious, I would look into 5 (or better 6) point belts, not only because it holds you in better, BUT it the alows you to wear a HANS.
Just my 2c, Andy