There is no answer to your query. It is entirely based on your preference. Al Brooks trades form his laptop. Steve Cohen of SAC capital has 8 monitors. It all comes down to your style of trading. Do you need to look at multiple charts? multiple indicators? How many instruments do you trade? etc. etc.
Yup, depends on trading style I believe. And personal preference on how to view a chart for example. I like one chart on a monitor generally, as I can see the history better. Second monitor might be the same time frame, same chart, just zoomed in so I can see just the last few minutes. Something you can't do very well if you don't have more monitors. Does it make you more money? I mean, does driving a Jaguar make you a much better driver? lol.
Looks like the USB video card idea isn't a very dependable one. I had my displays go off while it redetected the DisplayLink adapters in the middle of stuff I was doing. Unacceptable for trading, in my opinion. Also, with the usb display adapters it takes (I timed it) 4 minutes to finish booting, because it has to initialize each display adapter each time you boot. Initializing six displays from usb takes a while. Ok in a pinch maybe, but not ideal. I got a firepro 2460 and it boots fast now, and works pretty much as I'd want it to. I'm getting a second one to power the rest of my displays, though it will be a few days before it gets here. The 2260 adapters I got on ebay don't play nice with the 2460 by the looks, it wants both of them to run on the same driver, yet they use different drivers so pairing these didn't seem to work. I'll see how it is with two 2460's, but I expect they will work nicely together.
In case anyone is unaware of this, the Consumer Electronics Show in Las Vegas starts Tuesday. There will probably be announcements relevant to those of us who like lots of pixels. Also, Amazon, Tiger Direct and Sears are now selling the 39" Seiki 3840x2160 @ 30 Hz display for ~$500, with free shipping from Sears and for Amazon Prime members. It will be interesting to see if these prices drop even further if many cheap 60Hz 4k displays are announced at CES.
I'm interested in what hardware you are running... What is the box - what platform (brand, CPU, memory, etc.) and what else are you running inside it (raid cards, hard drives, optical or any other external devices? How is machine performance? (you don't have to tell us but I'm sure it must be a pain in the arse to move the mouse across all that - or you have the trackball speeds so high that the fine tune is rough) If that were me I'd be recommending three separate machines. One on each side (6 each) and one for the center two monitors.