I need to find out which is the better option 1. Connect my router to the server and then use NAT to share the Internet Connection between the other 15 machines or 2. Connect the single port router to my switch and let the other machines feed from the switch. What are the pros and cons of either approach as it relates to speed vs security?? I have an alcatel speedtouch 510 v4 single port gateway and 256/128 adsl connection

If your just going to use NAT for security using a server then it would be a poor choice compared to using the router to switch option for both speed and security. The reason is that if you use the server then your already allowing connections into the server which opens it up to possible hacking. Also, if the server is going to be performing other functions such as file share, or webservice, etc. then you are increasing it's workload by having it to process all security in and out of the network, slowing down peformance. Now if you were to ask if using the server with feature rich firewall services like Checkpoint, then that would be a different discussion, but NAT only then then your router do that.