The true way is that each core is at 5 Ghz and thats it. Sure it has a combined power of 40 GHz but no program on the planet can utilize that total processing power, maybe a few dual threaded applications can utilize a total of 10 GHz divided over 2 threads and a quad threaded application might be able to utilize 20 GHz spread over 4 separate threads but there are no practical octo-threaded applications let alone games out there right now that would be able to use the combined 40 GHz.
EDIT: I didn't even know quad sli was still commercially available, I thought nvidia tried and failed at that a while ago...