A couple of things to consider:
1) Did you press the CUDA switch after you set the processor? On the QS it is near the PRAM battery.
2) Did you ground yourself and the QS while working on it? Were there any static sparks? That would be the worst case scenario, that you damaged the mobo.
Since #2 would be fatal let's hope for #1! Put the dually in, press the CUDA button and hold it down at least 5 seconds. For good measure you can hold the ?-Option-P-R keys to reset the PRAM. A few chants and/or arcane gestures wouldn't hurt either.