Packet monitoring software

I've got a situation where I need to take a detailed look at what's going over a client's network. This is a first for me. I've been looking at Google and a mess of documentation for various firewall and packet sniffing packages for the last hour and a half, and all I've got to show for it is a bad headache and indigestion, so here I post :-)

My client is a small business with a network serviced by a cable modem for internet access.

I've got a laptop with Mandrake 10.2 Mandrivia 2005 LE on it. I've got two nics for the laptop. Ideally, I'd like to stick the laptop between the cable modem and the router, have the packets from the modem come in on one nic, get inspected and analyzed, and then the packet gets spit out unmolested to the router on the other nic. I want the reverse direction to work too. And ideally I don't want to have to wade through log files. I'd like a nice display that shows me an overview of what's going on live.

Are there any packages around, preferably free or inexpensive, that do what I want?