WPA encryption is about as safe as you can get for preventing attacks via wireless and preventing wireless sniffing. However, it will do absolutely nothing to prevent attacks via the internet, or the usual accidentally downloaded trojan horse, worms, viruses, and assorted exploits. That's where Norton Firewall helps. Your first line of defense is your NAT firewall/router. However, if that's deemed insufficient, a "personal firewall" such as Norton Firewall can be added to the obstacle course.
Think of the wireless link as just providing a replacement for a cable between your router and your PC. If the wireless were replaced by an ethernet cable, would you still be safe? You decide.