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.
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