Ubuntu Security :: Home Network Traffic Monitoring Recommendation?

I was reading a magazine article today which was a discussion of internet detective work for tracking down ip addresses which attempt an ssh login to your machine. I have never really paid much attention to network security since I only run a small home network. I have WPA encryption and a firewall on my router. But while reading this article, I remembered that I myself has seen log files in the past that inidicated someone somewhere had attempted to log into my machine (attempts all failed). This had happened a few times, but I never really considered it a threat.

But, the more I read about home computers becoming "zombies" for criminals, I guess I am getting a little paranoid in my old age, particularly since my wife does quite a bit of business on the net with credit cards. I have four computers connected to the net and each other on this network, and would like to be able to easily detect attempted log ins and deal with them quickly.

So my reason for posting is to ask if someone could recommend a novice-friendly application for monitoring traffic to check this intermittently. I have read bodhi.zazen's excellent tutorial on snort, but I it appears to be written for large lan's or web servers and is over-kill for a small home network.