Look up http://www.analogway.com for their Graphic Switcher II with their Widescreen AddOn, etc....you can download .pdfs showing example diagrams on the combination of Graphic Switcher IIs required depending on the number of screens you are projecting to. The downloads are somewhere on their site, if they have not changed their policy.
For software options, there are AVStumpfl's Wings Platinum with the TV/Video Module and Dataton's Watchout with a third-party VNC software add-on. Both require a WDM-compatible video capture card installed in each of the Slave/Display PCs. Both are meant for timeline (pre-programmed/pre-planned) playback, so might not be what you want, although each has the option to program/insert a jump to any other part of timeline allowing you to program a jump to a section(s) for live video(s), and jump back to pre-prepared graphics that loop. Matter of design, and preparation.
You might check out the Vista Systems Spyder, it's a multi display controller with alot of flexibility for playback formats. I think they come base level with 4 inputs and 4 discreet outputs. It's big brother, the Vista Montage, is the cadillac of high end multi screen control. The SPyder is supposed to be pretty economically priced. I'd look first at rental. I bet you'd find them at Scharff Weisberg in NYC, as well as at XL Video (NY,Atlanta,London,LA).
Wings Platinum and Watchout are very flexible, and relatively cheap, but as Thomas mentoned they are timeline based, so that can be a little kinky if you are doing live, extremely variable playback scenarios (although I have made them work in the past).
You could also consider certain of the new media servers that are sweeping the lighting world. The Hippotizer supports up to 3 discreet outputs (maybe 4 now with the new NVidia quad output cards), and High End Catalyst does up to 2 per unit I believe. You can run multiple's of either platform for more. They also have the benefit of being controllable by the lighting console, making flexibility and timing a real snap.
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