Giygas

It's not a matter of a best PC, but the right software. WinPcap would help a lot because of the easy promiscuous mode but when it comes to a PC, here's what I've done at some cafes before. This is extremely skiddy though, download Cain and Abel, APR poison (Man in the Middle attack) the devices you want and run a Wireshark scan. Traffic from the APR poisoned devices will be redirected to your PC and it will show the traffic on Wireshark. Wireshark commonly only displays your localhost traffic because the inability to go into promiscuous mode. Cain and Abel will be able to intercept that traffic information and display it. When you have wireshark running, to make it even easier, save the wireshark capture and use Network Miner to graphically symbolize the packet information. It will show the OS from the selected clients, the website and devices they interacted with. The images they've seen online, the sites they visited, etc. You can always download Backtrack linux, the wireless features are great.

Nothing fancy. I have a wired linksys BEFSR41v3 and a desktop (used for application servers and etc) connected to it on the first floor. The rest of my desktops (4 of them) are on the second floor so I have an ad-hoc network set up. They get their internet from shared internet from the server. Whenever I want to interact more directly and faster with the server and internet, I connect the dekstop I want to the router instead of the ad-hoc. I really need to buy a wireless router though.