Basicly I'd like to be able to switch through a few cameras connected via firewire and prerecorded video files and output on my second desktop which feeds projectors. On my primary or 3rd desktop have preview windows for each of the sources. And still have my primary free to type up titles for overlay. Basicly what I do now at home for watching dvd's/xvid and experimenting with my webcam. I just need some software to pull it all together seamlessly. As for having another gfx machine that could be done via tcp/ip, you can even do close to realtime streaming video over gigabit lan these days. Also I see no reason why it couldn't be done via firewire.
As for overtaxing the cpu, modern machines can handle alot of video before getting bogged down. This machine I'm on here can easily decode 4 xvid encoded videos at the same time and not show any signs of slowing down. I'm not looking todo any heavy realtime effects. I've had more problems with the few low end video mixers locking up than I've had with this PC. Now I do know what I'm doing with software, I know how to strip windows xp down so that only the nessary services are running, etc... Also I buy good hardware, server grade power supplies, motherboards, etc... My PC is as stable if not more so that alot of the dedicated products out there.
I'm in a really small market, this kind of production is only called on 2 or 3 times a year so going out and buying dedicated switches and scalers is out of the question. I'm just looking for an alternative to using composite video and bang boxes(The other guy in our market actually uses them).
As for the comparison to protools, I think someone is putting out a console that runs protools plugins and is targeted at live use. The main reason not to use a PC for mixing a band is the lack of IO and a control surface designed for live use. Take a look at a mackie TT-24, not much different than a PC, it just has the IO and control surface to do the job.