Yes, the technology is called "routing". You can use your computer as a router to route packets between two networks. For example, you hook up your computer to the Comcast cable modem, and on the other Etherport you hook it up to a switch. Then your computer #2, #3, #4, etc.. can share the same cable modem's bandwidth to access the Internet. These days the standalone routers and switches are readily available and are inexpensive. Very few people would set up the network that way.
avoid tradingcomputers.com ... in my experience, their stuff is overclocked, overpriced, underbuilt ... i found it to be junk
Almost any computer you can buy will have enough processing horsepower. Go cheap, if money is an issue, and spend the money on better/faster network connection (ie, colocate instead of DSL/cable).
I would go with Windows 7 Pro since its more stable. If you are trading futures, zentrader 64 with NinjaTrader 64 will actually work on a 64 bit system. I use just a laptop with 1 screen, but I could see where 2 screens would be better if you want to watch multiple instruments. Instead I just flip through a few to see which one is approaching a setup.
Boli, Routing is not the most common application of two ethernet ports on a server... typically this is used in a public / private network situation... such as having a server with one port facing the internet and the other facing the inside of a network. Routing *could* be enabled in the operating system, but most often it is not enabled. The outside server port allows you to place internet services like a web or email server on the public internet... while the inside port is used for management / database / storage area network purposes... note that many databases or storage networks are considered too valuable / insecure to be left with a direct connection to the internet. Another reason altogether is that you may want more bandwidth... perhaps you dedicate one ethernet to the storage network (which can consume nearly all the capacity) and the other for any purpose you like. For the purposes of the typical trader and the OP's question... as you mention, he probably will never use a 2nd ethernet port... PC magazine is probably thinking of the typical server application above, since you're looking at such a high-powered machine.
Have you had a bad experience with trading computers.com? I was thinking to buy from them or from ibuypower.com. Also, do you think 8 22" monitors on 3 video cards is too much?