GeForce GT 640 DDR3 vs GeForce GTS 250 1GB

Intro

Compare those specs to the GeForce GTS 250 1GB, which has a clock frequency of 738 MHz and a GDDR3 memory speed of 1100 MHz. It also uses a 256-bit memory bus, and makes use of a 65/55 nm design. It is made up of 128 SPUs, 64 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 GTS 250 1GB		145 Watts
	Difference: 80 Watts (123%)

Memory Bandwidth

Theoretically speaking, the GeForce GTS 250 1GB will be 23% faster than the GeForce GT 640 DDR3 overall, because of its greater bandwidth. (explain)

GeForce GTS 250 1GB		70400 MB/sec
GeForce GT 640 DDR3		57024 MB/sec
	Difference: 13376 (23%)

Texel Rate

The GeForce GTS 250 1GB will be quite a bit (more or less 64%) better at AF than the GeForce GT 640 DDR3. (explain)

GeForce GTS 250 1GB		47232 Mtexels/sec
GeForce GT 640 DDR3		28800 Mtexels/sec
	Difference: 18432 (64%)

Pixel Rate

The GeForce GT 640 DDR3 will be much (more or less 22%) faster with regards to anti-aliasing than the GeForce GTS 250 1GB, and capable of handling higher screen resolutions without losing too much performance. (explain)

GeForce GT 640 DDR3		14400 Mpixels/sec
GeForce GTS 250 1GB		11808 Mpixels/sec
	Difference: 2592 (22%)

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 data (in units of megabytes per second) that can be transferred past the external memory interface within a second. The number is calculated by multiplying the card's interface width by its memory speed. If it uses DDR type RAM, it should be multiplied by 2 again. If it uses DDR5, multiply by 4 instead. The higher the memory bandwidth, the faster the card will be in general. It especially helps with anti-aliasing, High Dynamic Range and high resolutions.

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

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