It won't over ride the old one by just popping it in... you would need to disable the old one in the Device Manager
(Right click on My Computer, choose mange, Click Device manager in pane to the left.)
I have 4 sound cards hooked up to my desktop.
1.) The soundcard on the motherboard (disabled)
2.) My modem is one of those cheap "winmodem"s that has an integrated sound card (disabled)
3.) An Audigy 2 that I use for my computers audio output
4.) My firewire Audio interface that I use for recording.
So, esentially, I am doing what you are talking about doing, but in spades.
Some of the sound cards built onto motherboards are descent enough, but you will probably be happier with a 3rd party audio card. There are some great, and fairly inexpensive, audio cards out there designed for recording that don't cost a whole lot more than a standard audio card (like an Audigy) Most of the audio cards designed for recording have monitoring functions built in (which, if I read correctly, is your main goal) I would keep that in mind if you decide to go this route.
