First you need to setup RivaTuner Statistics Server:
In order to implement it into wow, double click on wow.exe . You can edit the settings like u want with different colours, sizes, transparancy etc.
After you have done that, open HWiNFO64, go to settings -> OSD (RTSS) tab. Here you can choose whatever you hardware monitor you want to see in-game.
Click on the sensor u want to show in-game and press "Show value in OSD".
It works with any game, program and it can be very handy to see what hardware is bottlenecking you.