You need to look at the following things:
■1) Latency - Timings (lower is typically better)
■2) Speed - MHz
■3) Voltage - What is your voltage and does it meet your CPU's specs (Intel i3, i5, i7 less than 1.65V)
■4) Manufacturer - Is it a good manufacturer like Corsair, Mushkin, G.SKILL..
■5) Compatiablity - Is the RAM on the QVL list for your motherboard (not a must but is always nice!!)