Not long ago I stopped by a co-worker's office and caught sight of his set-up of four 24-inch Dell 2407 monitors, all connected to the same PC and all working together perfectly. Needless to say, I was impressed:
The four-monitor arrangement my co-worker was using was dual ATI x1900XTX video cards. He also has a PC with NVIDIA cards running the same array of displays. Pretty slick !
There is a lot of great hardware on the market today that can allow you to replicate this arrangement yourself -- all you need is a motherboard that supports two PCI-Express x-16 video cards.
For instance, AMD (ATI) has a solution called CrossFire™ that enables the use of dual video cards. Take a look at AMD's CrossFire™ website, which includes information for implementing this display array on your own desktop.
Not to be outdone, NVIDIA also has a solution that puts 4 video cards to use on a PC called SLI™. If you have a system running NVIDIA, get more information from SLI™ from their website.
You should note that many high-end video cards today are shipping with a dual-monitor option -- so while four monitors may be a bit of overkill, two might better suit you (and your wallet!). The good news is that Windows Vista is designed to natively accommodate multiple-monitor scenarios; for example, you can configure Windows Sidebar to appear on any connected monitor without first requiring preliminary adjustments.