GeForce 9800 GT 512MB vs GeForce GT 640 DDR3

Intro

Compare that to the GeForce GT 640 DDR3, which has a GPU core clock speed of 900 MHz, and 2048 MB of DDR3 memory set to run at 1782 MHz through a 128-bit bus. It also features 384 SPUs, 32 Texture Address Units, and 16 ROPs.

Display Graphs

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Power Usage and Theoretical Benchmarks

Power Consumption (Max TDP)

GeForce GT 640 DDR3		65 Watts
GeForce 9800 GT 512MB		105 Watts
	Difference: 40 Watts (62%)

Memory Bandwidth

Theoretically speaking, the GeForce 9800 GT 512MB will be 1% faster than the GeForce GT 640 DDR3 in general, due to its greater bandwidth. (explain)

GeForce 9800 GT 512MB		57600 MB/sec
GeForce GT 640 DDR3		57024 MB/sec
	Difference: 576 (1%)

Texel Rate

The GeForce 9800 GT 512MB should be a little bit (about 17%) faster with regards to AF than the GeForce GT 640 DDR3. (explain)

GeForce 9800 GT 512MB		33600 Mtexels/sec
GeForce GT 640 DDR3		28800 Mtexels/sec
	Difference: 4800 (17%)

Pixel Rate

The GeForce GT 640 DDR3 will be much (more or less 50%) better at FSAA than the GeForce 9800 GT 512MB, and also will be capable of handling higher screen resolutions without losing too much performance. (explain)

GeForce GT 640 DDR3		14400 Mpixels/sec
GeForce 9800 GT 512MB		9600 Mpixels/sec
	Difference: 4800 (50%)

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.

Price Comparison

Specifications

Memory Bandwidth: Bandwidth is the max amount of information (counted in MB per second) that can be transported over the external memory interface in one second. The number is calculated by multiplying the card's bus width by its memory clock speed. If it uses DDR memory, it should be multiplied by 2 once again. If it uses DDR5, multiply by 4 instead. The higher the card's memory bandwidth, the better the card will be in general. It especially helps with AA, High Dynamic Range and high resolutions.

Texel Rate: Texel rate is the maximum number of texture map elements (texels) that are processed in one second. This is worked out by multiplying the total number of texture units of the card by the core clock speed of the chip. The better this number, the better the card will be at handling texture filtering (anisotropic filtering - AF). It is measured in millions of texels in one second.

Pixel Rate: Pixel rate is the most pixels the graphics card could possibly write to its local memory per second - measured in millions of pixels per second. Pixel rate is worked out by multiplying the amount of Raster Operations Pipelines by the the core clock speed. ROPs (Raster Operations Pipelines - also called Render Output Units) are responsible for outputting the pixels (image) to the screen. The actual pixel fill rate also depends on lots of other factors, especially the memory bandwidth - the lower the bandwidth is, the lower the potential to reach the maximum fill rate.