i think a dual monitor can do you good. having more than that... i think you'll get confused later on.
hardware wise there are lots of video cards in the market that you can use.
on the software side, you'll just have to launch the software on each monitors.
aside from that, no matter how many monitors you install, you still got one set of eyes. you cannot work (or look) at all monitors at the same time, you still have to navigate your mouse on each monitor and work or look at it.
i use dual monitors, when i'm working on a software one side of the monitor is the application and the otherside is the visual basic application. as i modify the visual basic, i'll have to look at the other monitor to see it's effect. it'll be easier than pressing alt-tab all the time.
another suggestion instead of investing on many monitors, is that, get a bigger monitor, a 22 or 24" wide screen will be good.
if you need to read pdf files on one monitor, why not just print it instead, a piece of paper is cheaper than purchasing another video card and monitor, and consuming electricity.