X: Basically, Justin has given the info on LAN MIDI.Regarding the hardware route: just think of the slave computers as hardware synths,and you're near the mark. For example, on my DP 2.0 I run a single MOTU MIDI Express,giving me 8 ports.That is then connected to one of my AMT 8's.The only problem with that is V-Stack is capable of supporting 16x16,so to take full advantage, you'd have to double the number of MIDI interfaces.So,for that one computer alone you would require 4 MIDI interfaces. That's when hardware starts to become unwieldy. And expensive. I have five AMT/Unitors and a MOTU, but really don't wish to acquire more;so I'm going to take a look at MIDI over LAN. Maybe the answer is a hybrid configuration.
Regarding getting the sound out. Each slave computer has a soundcard: in my case the excellent Hammerfalls. I take the ADATs out of those into the Quad's rig of MOTU's. The audio i/o's on each are freely configurable in both the Hammerfall's software and the V-Stack's/Kore's and all other standalone instruments. It's way, way more powerful and flexible than the node method.
Hope this helps.