On adding a PCI-based Network Card to an existing Windows system, what will happen? Will it automatically take advantage of the extra network cards? I mean, what are the benefits of doing this?
Update: What about VMs? Can I route my VMs to use the second Network Card for all their traffic?
On a more serious note; an extra network card in any system (Windows or not) permits you to connect the system to an extra network; perhaps to serve as a bridge or router.
No... it will not automatically 'take advantage' of the card. Just like the first one, it has to be configured usefully to be of any benefit.
There is no benefit whatsoever if you don't already know why you might want one in the first place.
I don't think any operating system will 'automatically take advantage' of an additional network card. I certainly hope not anyway, because that approach will cause more problems than it solves.
Using more than one network card in a computer is perfectly possible of course, but it is one of the points at which networking becomes quite complicated, and as such you need to configure the computer yourself in order to support whatever it is you are trying to do...
When your windows system grows old, and you've gotten bored with it, you might repurpose the machine as a firewall/router using Smoothwall Express (open source) firewall. The two cards can then be used to a) interface to your ISPT; and b) connect to a router in your network. These systems are very user-configurable, and generate a ton of statistics and logs so you can be aware of how things work. Visit http://www.smoothwall.org/ for more info.
