Does using SLI to combine graphics cards essentially double your GPU power?

Additionally, do they have to be the exact same graphics card? (i.e. same manufacturer (EVGA...) and GT/GTX.

Does using SLI to combine graphics cards essentially double your GPU power?
yep in short it does but technically it depends on what your useing it for, getting twice the speed depends on how the game your useing it for works and how it is programed, newer games like crysis will deffinatly be able to use both graphics card but if the game was made for x-box then ported to pc like splinter cell double agent then you wont get double the speed but games like splinter cell and halo probably don't need that much speed. and yes they do have to be exactly the same.
Reply:SLI is short for Scalable Link Interface. SLI is proprietary to Nvidia graphics cards and is a method of allowing two graphics cards of the same model to be linked and share the load of rendering the graphics on screen. Using two graphics cards linked via SLI greatly improves the performance of the system by rendering the graphics faster. Often up to a 2 times improvement is seen.
Reply:The graphic cards must be the same and SLI compatible. Improvement is not fully doubled but an improvement of a minimum 70-80% is obtained not fully 100% doubled. And ya it takes a lot of power!! New power supply need to be bought, over 750W.