I have a 21.5" iMac connected to my basement ADSL Modem/Gateway via 25ft long UTP cable. Other devices (PS3, HP Laptop using Win 7, etc, etc.) are connected to the same ADSL Modem/Gateway via wireless. If wondering, this ADSL Modem/Gateway has its own firewall enabled and its application selection (for port forwarding) customized as well. If wondering, all works great. Especially after the iMac's MTU size was customized to 1496 (on its Ethernet Port). With above configuration in mind, should I also enable Security-Firewall within my iMac as well? Will firewall behind a firewall cause slow downs / problems, or create seamless "better" security? thanks.