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The most important rating, of course, is the price. Whatever performance one looks for needs to be within the affordable price range. Most reviews in the trade press try to address the ratings from this viewpoint. Some typical sources of review of graphics card are PC World, Tom's Hardware, CNET etc.
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Prices range all over the field. There are cards less than $30 all the way up to $850 and up. All the more reason to definitely fix on that price band you are will to shell out money for. That will decide what is the best performance you can buy. What follows is an attempt at explaining the performance parameters used by reviews. You should be able to easily decide which parameters mean the most to you.
Graphic cards are optimized for 3D/2D model rendering that is required for CAD based work and also in gaming. While the need for CAD based work is that the graphics card is able to render complex models and subassemblies accurately. Here the rendering of the model is the most important part. The time required to render is not all that critical. It does become critical when you want to do some kinematics studies, such as movement of these subassemblies. Then the redrawing of the models involved in the assembly will need to be done in real-time.
In a game scenario the real-time rendering of models has to be very quick. The animation used for games characters can involve very swift movements. The models in themselves may not be very complex, though intention is to make these models as realistic as possible. The more sophisticated the game, more high-powered graphics processing is required of the graphics card. Look up this article for a quick understanding of the important terms used in specifications of the graphics cards.
Objects in a given scene are created by a process known as rendering. Each object is broken up into a large number of polygons. If each polygon can be appropriately colored given the effect of lights and shadows then a given object is rendered realistically. Every time an object is moved somewhat all these color calculations are to be done all over again. That real-time rendering as in games calls for very high rendering power. The larger the depth of each pixel represented the finer the color you can display. Thus 24 bit color is very common today. Each of the 3 RGB colors are represented by 8 bits. Complex texturing of an object surface is often represented by texture maps. Overall the scenes represent a huge number of pixels to be handled. So the pixel related processing, the raster ops (ROP) and pixel pipelines are useful. Stream processors make operations on a stream of pixels easier. Large on-board memory helps the graphics cards handle large amount of data and relieve the main processor from working on them.
Other specifications that need to be looked at are if the card can be overclocked. Most gamers would like to push their cards to squeeze out the maximum performance out of them. Overclocking directly translates to more heat generation, and the basic card does a lot of processing and can generate a lot of heat. The cooling arrangement is an important specification. Physical form factor will tell you if you can accommodate the card easily inside the enclosure you have.
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NVidia GeForce 8800 GTX (768 MB) is a card rated high by editors as well as users. The GeForce 8800 GTX has a clock speed of 575MHz, and it comes with 768MB of DDR3 RAM, 900MHz with a 1,800MHz data rate. It has 128 stream processors, which can dynamically process information. When processing a shader-intensive scene, it can tap from more of the pipeline pool. It does not have to be limited to 24 or 48 pipes because some of the other pipes are set aside for geometry only.
An ATI graphics card was introduced on Sep 23rd. ATI Radeon 5800 graphics card is claimed to be the highest performing card in the world at 2.72 teraflops. FLOP is a measure of number of floating point operations done by a processor. A tera flop is a million times million floating point operations in a second. That is awesome processing power and should make high quality animation in HD resolutions easily possible. It also supports DirectX 11.. The picture shows the packaging necessary to manage the heat generation and corresponding cooling.