How can I monitor OTHER machines net/LAN traffic from my PC with ethereal? or prefferably tethereal? What would the command be? Or can I do it with nmap?
The reason im asking this is because I want to monitor net activity for PC A on my LAN from my slackware PC with some sort of a packet sniffer. How could I accomplish this? what apps or commands do I need?

Ok here we go, you can do any of the following.
1- Install Ethereal on the remote machine, and use VNC (client and server) to monitor the remote machine.
2- You can develop and software that uses libpcap to capture packets, and it can send send them to you via some udp sockets or so.
3- Do a mirror port on the switch where the remote machine(s) is/are connected and sniff packets out there.
4- If you don't want to sniff traffic in real time, then just run tcpdum and direct the output to some file on the remote machine, then use that file later. BTW back to metho number 2 you may use tcpdump and direct the output to some program (home made) that takes any input and forwars it via some sockets or so.

I use a package called iptraf on my internet server so I can monitor traffic in and out. I use a ssh connection to the server and then use iptraf from the cli