Opinion question

[Ualaa]

You can go into Task Manager.
Click on the Performance Tab.
Then click on Resource Monitor.

Once that is open, play wow as you normally do.
It will track CPU and RAM use, hard drive access, etc.
Which should give you an idea of what your current bottleneck is.

Upgrade whatever is holding you back the most.

In general, wow uses a lot of RAM.
Hard drive access is usually the bottleneck if you have enough ram.
Your CPU is very important, but generally not as much as the first two.
Video can be, if you have an older one, but for most of us this rarely holds us back - but a better card is more eye candy, which is very nice too.