I don't understand this kind of questions honestly. Isn't it normal that first you have to think what you want to do with the PC and then you see what kind of hardware is good for doing what you want? Like it usually isn't too hard to switch a video card and simply test if the other card is better for whatever you want to do with the pc...
Perhaps I am mistaken but this is the way I usually do things...
if you feel like there is something you want to do with but can't with the PCs you currently have then you should build something new in the middle.