As far a latency goes the system will usually try to clock them to the lowest common denominator, so it shouldnt matter so much. It likely wont matter anyway since when running 4 sticks you usually have to increase the voltage and/or ease the timings back to get all 4 to work. Also, this may have been fixed with newer boards, but I know in the past, the system would not run in dual channel mode with mismatching ram sizes.
Should work, notice the new memory is 2.2 volt, and your current memory is 2.1 volt. Your current memory has lower timings, so I would before installing the new, go into the BIOS and set the voltage to 2.2, and manually set the timings to what the new memory runs at, since it has slower timings. If you don't do this, you may not get the PC to boot if BIOS tries to automatically detect the voltage and timings and sets the voltage too low, or the timings to tight for the new sticks you are adding.