there are two flavours of rx 560. make sure you get the more powerful one
one has 16cu with 1024 stream processor and other has 14cu with 896 sp. 14cu one is gernally cheaper, but you won't know for sure unless you pay attention to the specs.
also, if your psu is very low end, get a rx560 without the 6pin power connector. the power consumption for rx560 is between 60~80w, so some model requires 6pin connector. to ensure minimal efforts, choose the model with no 6pin connector, it's also easier to install, just swap the cards and you are done.