Compatibility of VGA is dependent with motherboards. But if you have to slot, any VGA will go with any Motherboard where as, if you select a higher end VGA with PCI express 3.0 with a mother board which has only PCI express 2.0, the VGA will most likely to drop the speed of PCI express 2.0.
Whereas if you select a slow CPU with a high end VGA, you want be able to use the full potential of your VGA. Hence a well balanced setup may give you the best of both CPU and VGA.