I have 6 monitors, flat panel to save space. 2x19" for trading order entry - allows me to have 9 market depth windows up at any one time, and still have my orders, position, P&L, other markets overview, and announcements windows up at the same time.
4x17" monitors for my data provider. I use one monitor for news, trading calendar, quotes for the major markets, and 4 intraday charts. I use another monitor for 6 intraday charts. 3rd monitor is for quote boards so I can see the current price, open, high and low for all the main futures and forex markets, along with a short and long-term graph so I can quickly drag any market into it and see a price chart to give me the long-term perspective. 4th monitor is where I place quotes for stocks and worldwide stock indices I follow, and where I place my "trade shortlist" and "open positions" windows. I also use the 3rd or 4th monitor to enter orders via the internet e.g. IB.
Even with this screen space, I find myself flipping between windows. Ideally I would have 8 or even 10 monitors for intraday trading. Obviously for long-term stuff or just trading one market intraday this is overkill, 1 monitor is ok for that.