Check this out: My point is that if two similar cards have certain differences then SLI will be bottle necked with the lowest of both GPUs. I'm not deviating from the point that any brand can be used rather I used the word recommended. Here is an extract from Wikipedia on this issue which supports both arguments that any brand can be used and that one card with lower settings will also lower the settings on other card though the other card can operate at rather good settings:
Caveats[edit]
In an SLI configuration, cards can be of mixed manufacturers, card model names, BIOS revisions or clock speeds. However, they must be of the same GPU series (e.g. 8600, 8800) and GPU model name (e.g. GT, GTS, GTX).[5] There are rare exceptions for "mixed SLI" configurations on some cards that only have a matching core codename (e.g. G70, G73, G80, etc.), but this is otherwise not possible, and only happens when two matched cards differ only very slightly, an example being a differing amount of video memory, stream processors, or clockspeed. In this case, the slower/lesser card becomes dominant, and the other card matches. Another exception is the GTS 250, which can SLI with the 9800 GTX+, as the GTS 250 GPU is a rebadged 9800 GTX+ GPU.
In cases where two cards are not identical, the fastest card – or the card with more memory - will run at the speed of the slower card or disable its additional memory. (Note that while the FAQ still claims different memory size support, the support has been removed since revision 100.xx of NVIDIA's Forceware driver suite.[5])
SLI doesn't always give a performance benefit – in some extreme cases, it can lower the frame rate due to the particulars of an application's coding.[6] This is also true for ATI's CrossFire, as the problem is inherent in multi-GPU systems. This is often witnessed when running an application at low resolutions.
So dears, no one is wrong, I may no be able to clear my point of view but I hope it is clear now.
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