bry139 setup. I just got a PIII, 733, CDWR,
128/40. Before this I used a Mac 637CD running music shop (a way low end version of studio vision) only for sequencing.
This was the set up
Mac, Asr 10, misc midi gear, midi patch bay

Mac->modem port->midi in/out device:
Take midi IN computer to OUT on keyboard controller.
Take midi OUT computer to IN midi patch bay.
Patch bays midi OUT's to midi gear.
If you have more than 4 midi pieces it may be wise to invest in a midi patch bay rather than running a bunch of "THRU-IN-THRU" links.
My setup is not as saavy as many of my friends get-ups. But it will get you started.

With the PIII it is pretty much the same deal. Controller OUT-> PC IN
PC OUT -> midi bay -> midi gear. The midi i/o is on the sb platinum live drive.