I'm planning to buy Kingston's HyperX Blacks (1600mhz DDR3), but some sites say that my CPU's supported memory is: DDR2-1066, DDR3-1333. Does that mean that my desired RAM and my CPU will not be compatible? I'm ok if I only need to underclock the RAM, but will my PC boot at all when the speed is factory set to 1600mhz?
"factory" speed of your memory is "0". Memory is sold at the highest rated speed it is warrantied to work at, not the speed it will be running when you plug it into the board. Your motherbosrd determines that. 1600 mhz is actually an overclocked setting that your board may or may not have. If it does not, the memory will most likely be runninig at 1066 or 1333 when you plug it in.
1600 will work just fine in your board. If its cheaper, get it. But, look at the timings. 1333 mhz memory that has lower timings would work out faster in your board, since the memory is going to be running at 1333 any way.