2 Lan cards in a single CPU unit

Yes it is.

You would have this if use your computer as a router\gateway for any other pcs you have in your house\business.

Yes it's very easy to do but there is only one possible problem you [b]Can Not[/b] mix the type of LAN Cards so you can have 2 10/100 T Base LAN Cards or 2 Gigabite LAN Cards but not a 10/100 T Base & Gigabite LAN Cards as they will not work together.

Col

Yes.

Depends on what you are trying to achieve however.
You can mix the cards if you are accessing different networks. If you want some kind of team / failover then they must be the same.

Most windows versions only work correctly when one default gateway on one LAN card. You need some kind of routing to achieve any more. Like Routing & Remote Access in 2000 / 2003. You effectively turn your Computer in to a router.