can u key map sounds on a midi controller?
Well, MIDI controller is only MIDI controller. To do map the key you need software with that feature. Excuse me, but only if you can be more specify on what your goal and what's your problem, then may be I can get better view.
why would i need a soundcard with more than one midi slot?
For basic simple MIDI connection, one could be enough for you. If you kinda take more advance with MIDI, and about to put several synth / MIDI devices to be controlled, then you may end up need several MIDI port out of your PC.
Stealthtech had given you good idea to use sampler. For beginer, I would recomend using Soundfonts (you can use it with SB Live! / Audigy card). Fair quality for the price. It's good anyway. After you master it, then you can move to better sampler.
How do I start, Jaymz ?
All you need is Creative's sound card (either SB Live! or Audigy), MIDI kit (a pair of cables to connect your soundcard to your keyboard controller), and the keyboard controller. Then you'll need MIDI sequencer program to sequence & working with MIDI...
How do I get the sound, Jaymz ?
Load the soundfont from either your soundfont utility (comes with SB Live! card CD installation) or your soundfont supported MIDI application (eg. Cakewalk, SONAR, etc...). Do the setting within the software, and you're ready to rock. How to do that in detail... well, we can talk all day long. I think that's enough for now...
Jaymz