I have 4GB 1600 MHz RAM (7 7 7 20) 1.7V and want an upgrade.
Is there any difference to add 4GB 1600 MHz RAM with with different timings (9CL) and voltage (1.5V)?
Is there any pattern in what I can add to that or not?
Getting memory stick with different timings will only really matter if you truly care about losing a small fraction of a percentage point of performance.
Generally the only people who care that much are the people who are going to be overclocking their rig into an early grave. People who just want to get reasonably good performance and long life out of a computer very rarely notice any difference between already pretty marginal (tiny fractions of a second) memory timing differences.
Get what you can afford; lower timings are better, but don’t bother spending anything more than a couple of dollars/pounds/whatever-your-local-currency-is to get it unless you really care about getting half a frame per second more in the latest shooter.
Other than that if you are adding to your current RAM and want to use a new stick or two alongside it then you will want to get memory with the same timings as it is likely to work better together.
Thats not an upgrade other than the lower voltage so a bit cooler running. The lower the better regarding timings. Two years ago seems to be the peak of DDR3 ram performance wise, technology of the chips has not progressed at all since then. The price has dropped dramatically as has the voltage and the tradeoff has been performance. I myself am running DDR2000 @ 8-8-8-24 1.65v Kingstons HyperX which are two year old sticks, and absolutly nothing even comes close nowadays.