Multiple NIC cards per computer, can I use all of them?

I have two WinXP computers. Each has two 10/100 network jacks.
Currently each computer is connected to the router with by one network
jack. Could I increase my network performance and speed by hooking up
both network jacks?

Would I need to give each network connection it's own 192.168.0.x
address? Would I have to specifically make certain applications use
one or the other to balance out the traffic, or would WinXP take care
of that?

Thanks so much.