If your graphics card supports--and has enough ports for--the number of monitors you want to set up, excellent. If not, you may need to purchase an additional graphics card to get the multi-monitor support you are looking for.
Alternatively, newer monitors with DisplayPort multi-streaming support can be daisy-chained together from a single DisplayPort 1.2 connection on your graphics card, using additional DisplayPort cables to connect the additional monitors to one another. The various displays do not even need to be the same size or resolution.
Before you buy extra graphics cards, you will need to make sure that you have enough space in your tower (and open PCIe slots), as well as a power supply unit that can handle the extra strain.
If you buy a graphics card solely for the purpose of having multiple monitors, it is best to get one that is the same as (or, at least in the same product family as) your current graphics card, so you can connect them using SLI (Nvidia) or CrossFire (AMD). SLI and CrossFire setups will help your graphics cards run smoothly, and they will also boost your PC's overall graphics performance so you can do fun things like play games in multi-monitor mode without framerates plummeting. You will get much better performance with multiple connected graphics cards than you will with multiple non-connected graphics cards. And, while you technically can run Nvidia and AMD cards side-by-side...it is more trouble than it is worth and I do not recommend it.
Monitors, ports, and cables
Once you figure out your graphics card situation, it is time for the fun part: obtaining extra monitors. Extra monitors can be had for fairly cheap these days. Assuming you can not finagle a hand-me-down, a 24-inch monitor will run you around $170, while a decent 27-inch goes for about $250.
Of course, the perfect monitor for you depends on multiple factors, including the monitors you already have, the size of your desk, and what you are planning on using your extra monitor for.
In my case, I already had two 24-inch monitors, and I wanted a larger monitor as the centerpiece of my setup, so I picked up a 27-inch monitor and placed it between my two 24-inch displays. I am not using my multi-monitor setup to play multi-monitor games, so the difference in sizes (and the difference in heights--my 27-inch monitor's stand holds my 27-incher about one inch higher than my 24-inchers) is not an issue for me. However, if you are planning on doing a lot of gaming or watching videos that span multiple monitors, this height difference will make for a not-so-seamless experience.
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