the 6pin hookup is for a pcie express power plug from the PSU. some of the more powerful GPUs needs that extra power to run. SLI does not affect if GPUs needs it. for example, 6600 can do SLI, and niether card will need the extra power plug
The only cards that need that are the 6800 Series (including the ultra) and the geforce 7 series. The 6600 series dont require it. This is for nVidia, as im not sure about ATi since I dont follow them anymore.
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