Will a stock Q6600 (4x2.4Ghz) have enough horsepower to run Quad SLI on an nvidia 680i motherboard??  

 
Does Howdy Dudey have wooden balls?  
 
 
 
 
 
 
 
 
 
 
It should easily handle it.

Since when is there quad SLI on the 680i anyway?  Only 2 of the PCIe slots are full speed and the other is meant for a graphics accelerator.  
 
What were you planning to do?

he'd use 2x 9800GX2
 
2x2=4 obveiously

 
If you mean dual 9800GX2s, hell no! Reviewers are using 3GHz QX9650s or 3.2GHz QX9770s and its still showing signs of bottlenecking (sometimes on even a single 9800GX2, let alone 2 of them).
 
The good news is that Q6600s can overclock to 3.4GHz+ itself, you know what to do.    
 

Depends on your resolution.

Seriuosly, I saw a  9800GX2 review where it got oced pretty far and the Q6600 started bottenecking it so for SLI you'd probably want an Extreme edition CPU.

Wow thisll be one hell of a powerful machine

The bottleneck may exist, but the processor is not the only thing holding it back.  In fact your system memory, and chipset in general is going to be running at its max to keep it running.  you wont be unhappy with the performance, but there will be a better combination in the future to run that kind of craziness.  
 
yes, howdy dudey does have wooden balls.  I SEEN IT.