Selecting a video card upgrade can be an intimidating task. Unless you've kept up with all the GPU announcements and performance reports, it's practically impossible to know which cards are worth buying. Graphics processing units, like CPUs, improve year after year, and that means there's a staggering selection of graphics cards available to choose from and retailers just love to place obsolete cards right alongside the latest and greatest. If you're not careful, you could very well end up paying a lot of money for technology that's already a generation behind. Here are 10 things you need to know about video cards before shopping for one.

1. Memory isn't everything

Having 512MB of memory isn't going to help this Radeon X1300. You'd do much better with a 256MB Radeon X1600.

Here's the deal. You need a video card that has a decent amount of memory to play games at high-resolution with quality graphics settings enabled. Good video cards usually have lots of memory because all of that GPU horsepower will go to waste if you don't have enough memory space.

However, the video card manufacturers know that novice buyers look at memory size as one of the main comparison points between different cards, and that's why it's very common to see cards with cheap GPUs sporting 256MB or even 512MB of memory, which is sort of like dropping a 110-horsepower engine into the body of a muscle car. The underpowered card might have some of the right numbers on the spec sheet, but its poor performance will show once the gaming starts.

2. It's all about the GPU

Memory is important, but the real heart of the video card is the graphics processing unit. When you're browsing through video card names, the most important thing to look for is the GPU type, since that little chip is responsible for all of the video card's 3D performance. Today's best GPUs come from Nvidia and ATI, but it's not enough just to buy a video card with a "Nvidia GeForce" or "ATI Radeon" GPU. You also have to pay attention to the model number since Nvidia and ATI label all their cards from the sub-$100, entry-level cards to the $500 high-end monsters with the same GeForce and Radeon brand names. Higher model numbers are better, but you should also pay attention to additional modifiers at the end, such as GT, GS, GTX, XT, and XTX, since they often reveal important shader and clock-speed information. Study a few video card reviews or game performance guides to get familiar with the current models to see how they compare.

3. Pipelines, shaders, and clock speeds

The GeForce 7900 GTX has 24 pixel pipelines.

You could look at a GPU's clock speed and the pixel pipeline count to get a rough idea of the card's performance level in the early days of 3D acceleration. Today's GPUs have evolved to do much more than brute-force pixel processing. Lighting and other effects that used to take several pipeline "passes" can now run though a shader program to get the same results with fewer passes and less wasted work. GPUs now have specialized processing units dedicated to crunch through complex vertex and pixel-shader programs. Shader units might become an important specification to watch in future video cards as games become more shader-intensive. ATI has recently started reporting the number of shader units it has assigned to each pixel pipeline in its Radeon X1900 XTX line.

For the time being, you can still judge current GPUs by the number of pixel pipelines they have. GPU manufacturers also report vertex pipelines, but we haven't seen any games that bottleneck at the vertex-processing level yet. Entry-level cards usually have four pixel pipelines. Midrange cards have 8 or 12 pipelines, and high-end cards have 16 or more pipelines. Higher clock speeds are always better, but if you're choosing between pipelines or clock speeds, it's usually better to select more pipes over more MHz. Having eight pipelines running at 400MHz is much better than having four pipelines running at 500MHz.

4. Windows Vista and Direct3D 10

Microsoft plans on shipping its newest Windows operating system, Windows Vista, in early 2007. The new OS will feature DirectX 10, an updated collection of functions that software applications can use to access various system resources, including the 3D graphics card. The new version of DirectX incorporates a new version of Direct3D designed to streamline the graphics pipeline by reducing CPU overhead and moving more work to the GPU. Windows Vista will still work with current DirectX 9 video cards, but you'll need a DirectX 10 video card to run DX10-enabled games at the best settings.

We expect Nvidia and ATI to ship their first DX10 cards in the second half of this year, but you don't need to rush out and get one if you're afraid of game-compatibility problems. Game developers understand that it will be several years before the DX10 installation base surpasses the DX9 installation base. All games, including Vista exclusives Halo 3 and Shadowrun, will be DX9 and DX10 compatible for several years after Vista's arrival.