Here are a few key terms that can help you become a smarter shopper.
DirectX 11/DirectX 11.1
DirectX is the programming interface that most Windows-based PC games use. All of the latest graphics cards support the latest DirectX 11 API. If a card supports DirectX 11, it necessarily supports a bunch of features. For example, a card's packaging may mention "hardware tessellation" as if it were an exclusive feature, but all cards that support DirectX 11 must also support hardware tessellation.
This term refers to the graphics card's ability to generate additional game geometry, making characters and other objects with lots of curved surfaces look smoother.
Using antialiasing (AA) is still one of the best ways to improve a 3D image. Antialiasing blends the colors of nearby pixels and smooths the contrast between them. Doing so minimizes the classic "jaggies" effect that you may sometimes see in diagonal lines during gameplay.
GPU makers have implemented increasingly sophisticated antialiasing methods, including ways of blending pixel colors over time. Nvidia's TXAA is one example of this.
SLI (Nvidia) or CrossFire X (AMD)
If you want to run the highest AA levels on displays with resolutions of 1920 by 1200 or larger, or if you want to run stereoscopic 3D, as with Nvidia's 3D Vision, you may have to install two cards instead of one. The two cards are connected to each other via a short data cable so that they remain synchronized and effectively behave as a single graphics card with nearly double the performance.
DVI, DisplayPort, and HDMI
These are ports used to connect monitor cables. If you must connect via VGA--which I strongly advise you not to do--you'll need a DVI-to-VGA adapter. Most retail boxed cards include such an adapter.
DisplayPort and HDMI pose their own challenges. Cards may use mini-HDMI or mini-DisplayPort connectors. If the vendor doesn't include adapters in the box, you'll have to hunt them down on your own.
DVI-Dual Link connections can drive very-high-resolution 30-inch displays, but not all DVI connectors are created equal. Single-link connectors max out at 1920 by 1200, so check the product information if you need more.
HDMI's latest version is 1.4a, and you'll need that if you want to run 3D Blu-ray movies or to run 120Hz monitors in stereoscopic 3D.
DisplayPort is now at version 1.2, and most of its coolest features as yet go unused. DisplayPort 1.2 will permit you to daisy-chain two displays off a single port, but no monitors with DisplayPort 1.2 support currently exist, though they should begin to arrive before the end of 2012.
Like most tech products, graphics cards are segmented by price. Expensive cards tend to be more capable; and less expensive ones usually offer lower performance, consume less power, and are smaller in size (and so fit into a greater range of PC cases).
$400 and Up
The highest-priced cards deliver the strongest graphics performance, but they're also more power hungry. Both Nvidia and AMD have released a new generation of GPUs that are more power-efficient than their predecessors, but you'll still want a good 600-watt power supply to run these types of cards. At the extreme high end are dual-GPU cards, such as Nvidia's GTX 690. You can expect to spend close to $1000 for one of these.
One Nvidia GeForce GTX 680 or AMD Radeon HD 7970 should be able to run most games at high frame rates on 1080p monitors with settings at very high. Some games may show frame rate stuttering at ultra detail levels, so you'll need to test each game. If you have a high-end, 30-inch display running at 2560 by 1600, you'll need to manage your detail settings more carefully, unless you have a dual-GPU card, like the GTX 690, or are willing to install two discrete graphics cards.
$300 to $350
Cards in this category include the Nvidia GeForce GTX 670 and the AMD Radeon HD 7950. They offer performance that is nearly as good as that of the high-end models, but at a more modest price. If you stick with a 1080p or 1920 by 1200 display with one of these cards, you should be in good shape.
$200 to $300
At this level you may have to start making some sacrifices in detail settings. For GPUs such as the AMD Radeon HD 7870, that means running at "high" rather than at "very high" detail level, and it almost certainly means disabling antialiasing.
$100 to $200
Cards in this price range will run most games at modest detail levels, though in some cases you may have to dial back resolution as well. For example, if you have a Full HD monitor, you may want to fall back to 1680-by-1050-pixel or even 1440-by-900-pixel resolution to achieve playable frame rates.
These low-end cards are fine for most mainstream, GPU-accelerated applications, but their utility in gaming is pretty limited. For many current-generation PC games, you'll need to dial back detail levels to their lowest settings to get good frame rates.
Most recent graphics cards ship with more than 1GB of video memory. If you have a mainstream, 1080p display and don't use antialiasing, you probably don't need more than 1GB, but if you turn up the game's quality settings, you'll want more. Perhaps the most memory-hungry game I've encountered is Shogun 2: Total War, which can easily consume most of a 2GB frame buffer if you dial up all of the detail and AA levels.
When shopping for a card, be sure to pick a dealer that has a good return policy. Even though there are few incompatibilities, and the reliability of graphics cards is pretty good, it's reassuring to know that you can return a defective or incompatible card without incurring a restocking fee.
Warranties tend to vary. If you typically hang on to your graphics card for years, you may want to spend a few dollars more to pick up a card with a long or limited-lifetime warranty.
After buying the card, of course, you'll want to install it. PCWorld offers a handy guide to upgrading your graphics card. If you can turn a screwdriver, you can handle this upgrade.
With your new card installed and running, check out the latest crop of PC games. But beware: Once you get started on some of the immersive new titles that are available, you may find yourself still at the keyboard as birds begin announcing the coming dawn. That's when you'll know that you have both a great game and the right graphics card.