Firstly, the 2nd gen sandy bridge IMCs are dual channel controllers, not quad channel, so you can't expect 4 sticks to run in a quad configuration: rather the IMC has a dual channel pathway to each pair.
Secondly, manufacturers like to hype their memory speed specs, so they often quote DDR in terms of its "bits per second" rate; i.e. PC8500, PC10666, PC12800, PC14900 etc. With 8 bits per clock tick, the frequency (in Hertz) of each of these is 8500/8~1066MHz, 10666/8~1333MHz, 12800/8~1600MHz, and 14900/8~1866MHz, etc.
Thirdly, the memory is double data rate (i.e. DDR), and each channel is actually running at 1/2 the stated frequency; i.e. 1600/2=800MHz. But since there two data streams running simultaneously you get the equivalent of one channel at 1600MHz.
I'm impressed you got 4 sticks of 1600 MHz HyperX PnP running at 1600MHz with the G74! There are many threads around here with people who bought 16GB of that stuff or the 1866MHz variety and couldn't get the G74's BIOS to see it as anything but 1333MHz.
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G74SX-CST1-CBIL, i7 2630QM 2GHz
32GB DDR3 RAM @1333MHz
GTX560M 3GB DDR5 (192 bit)
17.3" LED 1920x1080
Sentelic TP, BIOS 203
Debian Linux Wheezy (Testing) Kernel 3.2, NVIDIA 295.40
