The more the merrier.
We observed from our own experiences from the Nvidia GeForce GTS 450 that the GF106 chip is quite formidable for the price – and the performance scales superlatively when paired with a twin GPU.
Now there's speculation that Nvidia could be plotting on releasing a dual-GF106 video card later this year. KitGutu said that it has data sheets on two GTS 450 cards, one rated at 75W and the second at 150W. Nvidia also listed two different GTS 450 cards in its driver.
While that's hardly conclusive evidence, it's certainly a nice hint that there could be a card coming for those who crave as many Nvidia GPUs possible in a system.