If you are playing 3v3 or 4v4 you may want a good CPU. Calculating sight, pathing, etc for hundreds of units will be CPU dependent. If your GPU doesnt handle the physics it may be offloaded to your CPU as well.
The major grind for COH2 is the density of alpha blended textures like smoke. Relic really upped the quality of these graphics meaning that more textures are being applied. The ground in a map can have several layers of textures that are blended. This means for the ground, each pixel is drawn 4 to 30 times per frame. Add some smoke on top of that and the pixels are being drawn an additional 10 - 50 times. Not just drawn, but blended with the current frame buffer. This WILL cause your FPS to drop to the ground. It is like asking your GPU to draw the game at 60 x (30+50) FPS. Put anti-aliasing in the mix and its just too much for most GPUs.
My guess is that a better GPU will always help more than a CPU. Then turn off AA and then lower physics quality until it doesnt choke
