Better use 2x4GB RAM's or 1X8GB?

Better to use 2x4 if you want to benefit from dual channel (faster access to the RAM). But using 1x8 will make it easier to upgrade in the future if you want to maximise your ram. (8x2(16gb), 8x4(32gb).
 
Assuming your motherboard supports dual channel memory, I would advise getting a matched pair of ram (2x4GB).

If you want to upgrade down the road to 16GB, you could always get a single 8GB now, and an identical one later (if you only have 2 memory slots).

But for fastest performance, make use of dual channel memory if it is available.
 


Of course you can buy a single stick of ram, were did you hear that you couldn't?