Yes, as Slack said,N-Track is the way to go with multitracking on the PC......
with Fruity Loops, i just export all my files as .wav files and bring them right into N-Track...its a piece of cake to do and you can keep each individual instrument (kick,snare,hihat,etc) on separate tracks in N-Track....
as for hooking up mics and instruments to the computer, you need some sort of preamp......your VS840 can serve that purpose, though standalone units are better...just plug everything into your VS, go from the outputs of the VS into the Audiophile.....play through it as if you were recording, only hit the red button on N-Track instead of the VS unit....simple as that......
keep in mind that along the way you may find additional costs associated with PC recording....when you get into higher track counts, you may need a faster hard drive....when you want tons of DX and VST effects your CPU may be a tad slow.....however, id say you probably have enough computer now to at least match what your VS does now.....and of course the PC will never beat the portability of the VS....
what are the specs on your computer?.....