Unsure if your looking to minimize space, or cost, or both, lol.

Not being able to see my alts windows (when i want to) drives me nuts. and lets face it, sooner or later, your main is gonna die and your gonna have to switch mid-fight. doing that with headless machines would just be yuck...

Maybe im biased (since i went whole hog and built a rack, lolz) but i'd say aim for minimal, but still capable hardware. (300-350 dollar range per comp). You can stack rackmountable cases without a rack (just put wood in between each case). could do the same thing with towers tho. and a stack of 4-5 comps doesnt take up that much space. Doing something like the guy that built a cluster in an IKEA cabinetis also an option. (kinda like what you were talking about with more than one comp in a case, but on a grander scale).

I hear shuttle pc's are loud, never used one tho. Course, people say that racks fans are really loud, but mine aren't, so who knows. eh. I just cant see building a comp without a 3d card LOL. If your going to run them with settings that low, well, you could probably get all 4 to run on one half decent secondary machine... I've never tried. I can't tolerate any lag whatsoever. lol. Also, this guy seems to want to get rid of very minimalistic pc's pretty cheap; might be in line with what you are talking about, if you somehow missed that thread.

Just my thoughts, if you do it, let us know how it works out, hehe. I sorta want to do a "Ikea Computer" but can't quite justify it.