Your example is flawed. It's not even close to the same thing. The case has no bearing on the performance of the CPU or GPU. The CPU on the other had as a direct and significant effect on the performance of the GPU, not to mention the entire system as a whole. I wouldn't want my $1200 GPU to perform like a $400 GPU becuase I chose to pair it with a $150 CPU. Now if you spend more on the CPU, your GPU will perform like it should. However, a case made of pure gold with platinum accents and diamond rivets won't do jack for performance.
Make sense?
Except I can overclock a $300 CPU to smoke a $1000 CPU. No matter how hard I try, I unfortunately can't overclock 1GB of VRAM to be 2GB of VRAM.