In general, if you open the Performance Monitor, you can view the strain your current game is putting on your hardware.
Anything that is close to 100%, or that hits 100% of an available resource, indicates something that is a bottleneck for your current system.

Upgrades to bottlenecks are where performance will increase.

Not really sure what the requirements will be for the next expansion as I've not been in WoW for a while now.
But... the performance monitor will show you what your bottlenecks are.