While idle/near-idle INtel's Speedstep (EIST) downclocks the CPU to 798MHz. The CPU will only run @ 2.9 GHz if one thread is loading one core: if all four cores are blasted by 4 or 8 threads from Prime95 the turbo frequency will be less (guessing 2.5-2.6 GHz) because of thermal considerations.
As well, its fairly well documented that the G74 running Windows does not allow the 2630QM to reach its full 2.9 GHz: check out this review (http://www.notebookcheck.net/Review-Asus-G74SX-3DE-Gaming-Notebook.56809.0.html) where the conclusion is "the G74SX obviously doesn't exploit the i7-2630QM's maximum possible clock rate when only one core is put under load". It seems to max out at 2.7-2.8 GHz. However, I do get the full 2.92 GHz from one thread with my G74 running Linux. The version of ACPI Thermal Management installed in the OS seemed to be the key to getting the CPUY to go at 2.92GHz; a newer Linux kernel did the trick.