247x12: FSB of 274, multiplier of 12x, giving you 2964Mhz or 2.964Ghz.
Voltage at 1.61v?: try checking it in the bios or another program like MBM5, or the monitoring program that came with your motherboard - not sure which one, maybe Winbond Hardware Doctor.
With the 1.33 setting, it will probably be your CPU limiting your overclock, as you say you were already running at 3.12Ghz, with no adjustments of the CPU voltage.
With the 1.66 setting it was probably your RAM limiting your overclock, as your CPU could go higher than 2.96Ghz.
If not that, then it was your power supply.
Would you mind listing what exactly is in your system? Except for your 2.4C, Gigabyte 875 motherboard, what type of DDR do you have, power supply, cooling etc etc...
also, get a program like Prime 95 to stress test.