Hardware

You don't need to buy a $2,000 machine to run wow, but at the same time you can't use your old 386 and expect it to run.

This page gives a little more information regarding hardware than the System requirements gives.

Notes if buying a new machine

 * 1) Cheap specials are usually cheap for a reason: the hardware will require upgrading to run WoW decently.  $400 computers usually require a graphics card (most have integrated which isn't recommended for wow), and then a new power supply to run the graphics card without frying the machine.
 * 2) It is advised to take whatever BestBuy employees state with a grain of salt - they have been known to tell users that Wow will run fabulously on a specific computer, when it will barely run it.

System Requirements
Keep in mind, on the World of Warcraft box, they list the minimum requirements to play the game, that means you're guaranteed to be able to load up the game without it warning you that you don't meet the requirements. It also guarantees that you will get at least 1 FPS (Frames Per Second) on the lowest settings possible. To achieve smooth gameplay, you'll want to have a computer system that is closer to the Recommended requirements also listed on the box - of course surpassing those specs are better if you play with a large resolution monitor.

CPU
There's a lot of speculation on this subject, a good rule of thumb is that the faster the clock speed, the better the game will run. A faster Dual-core processor can usually work better for World of Warcraft than a slower Quad-core. Single-core processors such as the Intel Pentium 4 and AMD's single core offerings will show their ages quickly with the game.

Recently, with the release of Patch 4.0.1 implementing graphics upgrades and other things, Blizzard has stopped supporting older CPUs such as older AMD Athlon XP's, and a lot of Intel Celerons. If your processor doesn't support SSE (Streaming SIMD Extensions) Instructions, you might be able to run the game, however, if any issues crop up, you're pretty much out of luck for any help.

Graphics
With graphics cards, all the different model numbers may seem confusing, but they're actually very easy to decipher for the most part.

The first number in the model is the Generation of the card, the remaining numbers are the relative power and capabilities of the card. So if you have an Nvidia 6800 and an Nvidia 7200, while the 6800 is older, it has more power, and can do more. With ATI Cards, it's the same thing; an HD 3850 will outperform an HD 4350.

You don't need to have a super powerful graphics card to play World of Warcraft, but having an integrated (built into the motherboard) graphics chipset such as the nVidia 6150, ATI's 4200, or anything from Intel's graphics chipset offerings -while playable- will ultimately result in poor fps as they just don't have the horsepower to handle everything that goes on in the game that well.

Also note that lately that the built-in Nvidia 9100 integrated graphics chips have been overheating, causing a massive FPS loss in World of Warcraft, ultimately leading to failure of the integrated graphics chip altogether. If you can catch the overheat in time (a symptom is lower than usual fps), you can set up a new fan in the machine pointed at the motherboard. However if the chip does die, the only fix for this is to purchase a REAL graphics card - preferably with a fan attached.

With the release of Patch 4.0.1 in preparation for the release of World of Warcraft: Cataclysm, Blizzard is no longer supporting the VIA/SG3 Unichrome graphics chipset.

Memory
Wow recommends you run 1gb of RAM on XP, and 2gb of RAM for Vista/Win7, however theres a noticeable difference in-game with how smoothly things run when running more RAM in Vista (ie 2 vs 3 vs 4gb of ram).

There's a noticeable stutter in-game on medium settings with 2gb in Vista with nothing running in the background.

Its recommended that you run 4gb of RAM, more if you multitask often.

Hard drives
This is a list of slowest drives to fastest - fastest is best for load times.

Notebooks
Notebooks MOSTLY run 5400rpm drives (some gaming ones now use SSDs). These are the slowest drives. Your load times will be longer than if you were playing on a desktop.

7200 RPM Drives
Unless you're playing on a notebook, you'll usually be using a 7200RPM harddrive. These are just fine for wow, and are quite standard.

Raptor Drives (10000RPM)
You can also use a Raptor drive, which will decrease loading times in wow.

7200 RPM Drives in RAID0
You can get approximately the same decrease in load times compared to Raptor Drives, by using 2 7200rpm harddrives, and putting them in RAID0 - however you risk if one dies, you lose everything on both drives.

SSD (Solid State Disks)
These are the newest drives on the market, and currently the fastest. They're expensive for the amount of space on them. If you're going to use one of these, you don't want to use the entire thing as storage - have your OS, and a game or two installed to it, and use a regular 7200RPM drive for everything else.