Well, it's not pretty, but I finally managed to hook up everything I care about right now. I'm in dire need of some real computer desks. But I'll probably live with things this way for a while. I may even grow to like it. I need an audio switcher. Right now, the output from everything's speakers ends in a (female) stereo mini-jack. I plug the speakers' (male) mini-plug into whatever I want to hear. You can see the VGA switchbox (next to the left speaker/rum & OJ), which unfortunately adds some ghosting to the (analog) signal. The good news is that the digital DVI output from the NVidia card in the gaming PC (right) works like a charm. So I use the monitor to switch from analog (cheap workhorse PC & Xbox 360) to digital (game PC). Awesome crisp image through DVI.
I was having some misgivings about the performance of this LCD monitor, but after seeing the NVidia desktop display, plus Doom 3 and Psychonauts, I can say for sure that the monitor performs very well. It benefits from a negative gamma adjustment, something the integrated Intel chipset on the cheap box doesn't allow.
I've seen a fancy switchbox ($100) which allows you to use multiple PCs with a single mouse, keyboard, monitor, and speaker set. I may look into that, although the price is steep.