personally, i'd just use one computer, i've read that the via's can output to 2 screens without too much problems.. mind you, you may need 2 video cards, and the via's usually only have one pci slot, so the audio might be a harder issue since the pc would need dual sound cards to be able to output dual audio if you wanted that..
but i doubt u'd need a p4, since they're a waste of power anyway.., could use an older athlon, maybe 1.4-1.5ghz, with 2-3 pci slots on the mobo, and u'll be fine
for the rear screens, if they're going to show the same thing, just use a video splitter
and there are lots of programs out that control what goes on what screen, "Ultramon" is one, check it out, might work for what you want, so ur gps set to the front monitor, and ur video program set to monitors 2 and 3..
as to having the front play video as well, just change the config, set the screens to clone