Monitors are important not because of their impact on performance, but rather their impact on the usability of a computer. A poor quality monitor can hamper the use of an otherwise very good computer, because a monitor that is too small or hard to look at can make the system hard to use. In many cases two or more monitors can also be helpful, as more information can be displayed at a time and two smaller screens are often more affordable than one larger model. If you are going to use multiple monitors you will also want to make sure that your video card setup can accommodate the quantity you plan on running; if you are unsure about that feel free to contact us at firstname.lastname@example.org.
There is also one category where monitors - or more precisely resolution, which is a characteristic of monitors - does directly impact performance: gaming. When playing a 3D game the higher the resolution is set the more work there is for the computer, especially the video card(s), to do. For the best experience you want to match the size of monitor you will be using with the right level of graphics card.