Compare any two graphics cards:
Radeon HD 3850 256MB vs Radeon HD 3870 512MB
IntroThe Radeon HD 3850 256MB has a core clock speed of 668 MHz and a GDDR3 memory speed of 828 MHz. It also uses a 256-bit memory bus, and uses a 55 nm design. It is made up of 320(64x5) SPUs, 16 Texture Address Units, and 16 Raster Operation Units.
Compare all that to the Radeon HD 3870 512MB, which has GPU clock speed of 775 MHz, and 512 MB of GDDR3 memory set to run at 900 MHz through a 256-bit bus. It also features 320(64x5) Stream Processors, 16 TAUs, and 16 Raster Operation Units.
Power Usage and Theoretical Benchmarks
Power Consumption (Max TDP)
Theoretically speaking, the Radeon HD 3870 512MB will be 9% faster than the Radeon HD 3850 256MB overall, due to its greater bandwidth. (explain)
Texel RateThe Radeon HD 3870 512MB will be just a bit (more or less 16%) faster with regards to texture filtering than the Radeon HD 3850 256MB. (explain)
Pixel RateThe Radeon HD 3870 512MB is a bit (more or less 16%) better at AA than the Radeon HD 3850 256MB, and will be capable of handling higher screen resolutions more effectively. (explain)
Please note that the above 'benchmarks' are all just theoretical - the results were calculated based on the card's specifications, and real-world performance may (and probably will) vary at least a bit.
Memory Bandwidth: Memory bandwidth is the maximum amount of information (in units of megabytes per second) that can be transported past the external memory interface in one second. It's worked out by multiplying the card's interface width by its memory speed. If the card has DDR memory, it must be multiplied by 2 once again. If DDR5, multiply by ANOTHER 2x. The higher the card's memory bandwidth, the better the card will be in general. It especially helps with anti-aliasing, HDR and high resolutions.
Texel Rate: Texel rate is the maximum amount of texture map elements (texels) that can be processed in one second. This is calculated by multiplying the total texture units of the card by the core clock speed of the chip. The better the texel rate, the better the graphics card will be at texture filtering (anisotropic filtering - AF). It is measured in millions of texels in a second.
Pixel Rate: Pixel rate is the most pixels the video card could possibly write to the local memory per second - measured in millions of pixels per second. The figure is worked out by multiplying the number of colour ROPs by the the core clock speed. ROPs (Raster Operations Pipelines - also sometimes called Render Output Units) are responsible for filling the screen with pixels (the image). The actual pixel output rate also depends on lots of other factors, especially the memory bandwidth - the lower the bandwidth is, the lower the ability to get to the max fill rate.