How to determine what video card to get with a computer purchase is heavily dependent upon what the computer is going to be used for. I tend to find that there are four categories that people can be placed in when it comes to computer usage and video card needs: casual computing, graphic design, light gaming and serious gaming.
Before going into detail about the different categories, I will mention that it is possible to upgrade the video card on most desktops. All modern systems should use the PCI-Express graphics card slot, also referred to as a x16 slot. There are several version of PCI-Express from 1.0 to 3.0. The higher versions offer faster bandwidth but they are all backward compatible so a PCI-Express 3.0 card will work in a PCI-Express 1.0 slot. Older systems used AGP but this has been discontinued in favor of the new slot. Be sure you know which your PC uses before buying one. Also, be sure to know what the wattage is of the computer's power supply as this will likely determine what kind of card can be installed.
Casual computing tends to be those tasks related to using the computer for word processing, web browsing, watching a video or listening to music. None of these tasks requires much video processing power at all. For this category of computing any choice of video processor will work. It can be integrated into the computer system or be a dedicated card. The only exception to this is high definition video such as Blu-ray. For this, an integrated solution rated for HD video is important but most now are capable of this. Now, many casual computing people may also want to do some editing of home videos for posting on the web or sending to friends and family. Those looking at this may want to consider getting an Intel based system with HD Graphics for support of Quick Sync Video which is very efficient at transcoding digital video. Of course, AMD's graphics integrated into their APU products offer a bit more flexibility in terms of accelerating non-3D programs.
Individuals looking to do graphic design or even video editing will want a few more features with the video card. For graphic designs, it is generally good to have higher resolution capability. Many high-end displays can support up to 2560 by 1600 resolutions allowing for more visible detail. Another feature that may be of interest is multi-monitor support. This allows the graphics card to support two or more computer displays at once expanding the graphical workspace. For extremely high resolution displays such as 4K Displays , you may be required to have a DisplayPort connector on the graphics card. (Note: Apple computers use a port referred to as Thunderbolt that is compatible with DisplayPort displays.) Check the monitor for requirements.
Photoshop CS4 and later users can gain benefits from having a 3D accelerator to boost performance. At this point, the boost is more dependent upon the speed and amount of video memory than it is on the graphics processors. It is recommended to have at least 1GB of dedicate memory on a graphics card with 2GB or more being preferred.
Individuals looking to do video editing also have additional needs to those of casual computing. This is especially true for transfering older analog video to the computer. They will want a feature called video-in/video-out or VIVO. This allows a video source to be plugged into the computer for digitizing of analog video sources such as VHS or older digital video tapes as well as exporting a video signal back to those devices. This feature is not common in many video cards today and is instead offloaded to USB based peripherals that are designed to digitize the video.
When talking about gaming, I'm referring to games that use 3D graphics acceleration. Games like solitaire, Tetris and Farmville don't use any 3D acceleration and will work fine with any form of graphics processor. If you play 3D games every once in a while or even on a regular basis and don't care about it running as fast as possible or having all the features to enhance the detail, then this is the category of card you want to look at.
Cards in this category should fully support the DirectX 11 graphics standard and have at least 1GB of video memory. It should be noted that Direct X 11 and 10 games will only fully work on Windows 7 and later. Windows XP users are still restricted to Direct X 9 features. For particular brands and models of processor, check out the selection in the Best Video Cards from $100 to $250. Most of these will be able to play games up to 1920x1080 resolutions typical of most monitors with varying quality levels.
If your next computer is going to be an ultimate gaming system, then you want to make sure that you have a video card that matches the capabilities of the system. It should be able to support all the current 3D games on the market with acceptable frame rates with all of the graphic detail features turned on. If you also intend to run a game on extremely high resolution displays or across multiple displays, then you want to look at a higher end graphics card.
All performance 3D video cards should fully support DirectX 11 and have a at least 2GB of memory but preferably more. For particular brands and models of processors, check out the selection in the Best Performance Video Cards. Be warned that if you are looking to add one of these cards to your existing desktop, make sure that you have the proper wattage power supply to support the graphics card.
While the primary focus for graphics cards has been on 3D acceleration, more and more applications are not being used to access the improved math capabilies of graphics processors compared to trafitional central processors. A whole range of applications are now written to take advantage of the GPU's capabilities for improved performance. I mentioned this somewhat in the graphics design section above, but there are a greater range of applications than just graphics. They can be used to help process data in scientific research such as Seti@HOME or other cloud computing tasks. It can help reduce the amount of time that it takes to do video encoding and conversion. It is even possible to use them for mining of cryptocurrencies such as Bitcoin.
The problem with these specialized tasks is that the choice of video cards is very highly dependent upon the programs that will be accessing the card. Some tasks run better on a specific brand of graphics card or maybe even a specific processor model from a brand. For instance, AMD Radeon R7 and R9 cards are generally preferred for those doing Bitcoin mining thanks to their improved hash performance. NVIDIA cards on the other hand tend to perform better when it comes to some scientific applictions like Folding @ Home. As a result, try and do reaseach into any heavily used programs before selecting a video card to make sure you are getting the best fit for your need.