I have recently bought HP envy 4 ultrabook which came with 4gbX1 module of hynix ddr pc3 12800 @ 1600 Mhz, 204 pin and CL=11, non ecc, unbuffered at 1.35Volts (second slot is empty). I plan to upgrade RAM to 8gbX2 module. Can you suggest me which RAM module i should buy. Laptop supports maximum ram of 16gb. I have win 8 which is 64 bit system