Check with your organizations as to what Snell rating they require. I've not done any autocross or DE in the Boxster, but for karting I've never come across a situation where my M-rated helmet wasn't acceptable. Honestly, the biggest difference between M and SA is in the fireproofing, and if that was such a concern they'd require everyone to have a firesuit..
That said, I've got my eye on a new helmet, probably for next season, and I'll be going with an SA because I eventually have my eye on doing some open wheel at some point and I want to be sure what I buy for karting I can use for the big cars.
I've got an HJC now - it was cheap, it fits well, and is well ventilated. I also like the Bell helmets, they're priced well and have a great reputation. I can't comment on the G-Force helmets, but I've got a pair of their driving shoes and they're well made.
Make sure if you don't have a local shop where you can go try them on that you understand and adhere to the return policy. All helmets fit differently and you want to make sure you can return it and that you understand what you can and can't do before you return it. I've heard of people not being able to return a helmet because they took the plastic film off the visor.
One last recommendation.. If you can try to find a helmet with a removable and washable liner. One summers' worth of karting in hot weather and that helmet starts to become an uncomfortable place to be with the shield down
Also, if you're shopping online, I recommend saferacer.com
I've bought everything but my helmet through them (bought that through a local motorcycle shop) and I've had nothing but good experiences.