GeForce 9800 GX2 vs GeForce GT 640 DDR3

Intro

Compare all that to the GeForce GT 640 DDR3, which features a clock frequency of 900 MHz and a DDR3 memory speed of 1782 MHz. It also makes use of a 128-bit memory bus, and makes use of a 28 nm design. It is comprised of 384 SPUs, 32 TAUs, 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 GX2		197 Watts
	Difference: 132 Watts (203%)

Memory Bandwidth

In theory, the GeForce 9800 GX2 will be 124% faster than the GeForce GT 640 DDR3 overall, due to its higher data rate. (explain)

GeForce 9800 GX2		128000 MB/sec
GeForce GT 640 DDR3		57024 MB/sec
	Difference: 70976 (124%)

Texel Rate

The GeForce 9800 GX2 is quite a bit (about 167%) faster with regards to anisotropic filtering than the GeForce GT 640 DDR3. (explain)

GeForce 9800 GX2		76800 Mtexels/sec
GeForce GT 640 DDR3		28800 Mtexels/sec
	Difference: 48000 (167%)

Pixel Rate

If running with a high resolution is important to you, then the GeForce 9800 GX2 is the winner, by far. (explain)

GeForce 9800 GX2		19200 Mpixels/sec
GeForce GT 640 DDR3		14400 Mpixels/sec
	Difference: 4800 (33%)

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.

One or more cards in this comparison are multi-core. This means that their bandwidth, texel and pixel rates are theoretically doubled - this does not mean the card will actually perform twice as fast, but only that it should in theory be able to. Actual game benchmarks will give a more accurate idea of what it's capable of.

Price Comparison

Specifications

Memory Bandwidth: Memory bandwidth is the maximum amount of data (counted in megabytes per second) that can be transferred past the external memory interface within a second. It is worked out by multiplying the card's interface width by the speed of its memory. If the card has DDR memory, it should be multiplied by 2 once again. If DDR5, multiply by 4 instead. The better the memory bandwidth, the better the card will be in general. It especially helps with anti-aliasing, High Dynamic Range and higher screen resolutions.

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

Pixel Rate: Pixel rate is the maximum amount of pixels that the graphics card could possibly write to its local memory in a second - measured in millions of pixels per second. The number is calculated by multiplying the number of Raster Operations Pipelines by the the core speed of the card. ROPs (Raster Operations Pipelines - aka Render Output Units) are responsible for drawing the pixels (image) on the screen. The actual pixel rate also depends on quite a few other factors, most notably the memory bandwidth - the lower the memory bandwidth is, the lower the potential to reach the maximum fill rate.