With the SB you should be able to record at least 2 tracks simultaneously using the card's stereo Line In input.
You will need a Y connector to do this. One end will need a stereo mini-plug to go into the sound card, and the Y branches will need mono connections suitable for what you are planning to plug in e.g., 1/4" phone (guitar) jacks. Take a trip to Radio Shack. They should be able to fix you up.
I believe the SB Live also has SPDIF inputs as well, which might allow you additional inputs beyond two, but these are digital and get a bit trickier to use.
You will also likely need some type of pre-amp to boost your signal to line level (for a guitar, you can use a POD, or possibly a line out connection off your amp).
I would not recommend using the Mic In. That will force you to use the SB's pre-amps, which are pretty crappy.