"The speed of transfer between the two GPU's in a 512 card is much faster then any transfer rate between two 256's. So, therefore, why would it ever be faster? "
A pair of 7900GTs in SLI vs a single GTX512...(not sure what you meant by "the gtx512's two gpus" comment....; are we not discussing SLI 7900GTs vs a single gtx512?)
When using split frame rendering at 160x1200, essentially each 7900G is rendering 800x600, which will be faster than a single GTX512 can render 1600x1200...
SLI performance varies from game to game, but many games receive a 40-50% boost form the 2nd card, and some (FEAR) as high as 80%....
SLI 7900GTs is rather expensive, however, compared to the $399 that a single 1900XT is available for....