If you notice, that's showing information for Core #0, not both combined. On the lower right you'll also notice where it says, "Cores: 2".
So, 1199.9 x 2 = 2,399.8 = ~2.4GHz.
This is incorrect. The CPU is a dual core and both cores run at a maximum of 2.5GHz when under load. But when the CPU is idle, Speedstep is active and lowers the speed to its lowest possible based on the multiplier, for which yours is 6x. The maximum multiplier yours can do is 12.5x which at a 200MHz FSB would be 2.5GHz per core.
You can disable Speedstep in the BIOS but there is really no need. I have a Q6600 that runs idle 2GHz but under load goes to 3GHz. And from what I have seen, it clocks to its top speed when under about 20% load.
For the most part, when your system needs it it will clock up to 2.5GHz. So when gaming or running a CPU intensive application it will stay at 2.5GHz until you close it.