I think a Mac computer is the best choice because all of the most popular software available for the PC is also available for the Mac,

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Final Cut Pro is only available on a Mac. Additionally, programs like Pro Tools add to your abilities for producing a quality product and Apple itself is dedicated to home video and audio production.
Pro Tools often will require an entire system on its own to run properly, so this is up to how much you want to spend and whether or not you want multiple systems.
If you are dedicated to the PC then computers that are designed for digital modeling are the best choice. These would be computers that are used by architects or large-scale urban developers for 3D imaging. Computers also used for home gaming are great because it will guarantee that you have the newest graphics and audio cards. Alienware makes great PCs that are capable of handling the workload needed for video editing, especially their Area-51 ALX. The best way to get a PC that will handle editing well is to have it built to your specifications. An ASUS motherboard is a good choice, and nVidia Quatro or Geforce graphics cards are great because of their ability to support gaming. Make sure to have a large enough hard drive (over 160GB), with plenty of RAM (more than 4GB), and a quality sound card. I recommend you go with the Creative brand cards.
If you are going to use a Mac, the best choice is going to be an iMac or Mac Pro built with all the optimum specifications you are going to need. Make sure to get the latest version of the Mac OS X, which is also known as Leopard, and try to get a second monitor. This should not be too expensive and will make the editing process much easier. An iMac may be fine for a lot of editing, but a Mac Pro or Macbook Pro may be important for larger projects.