CPU vs GPU Mining (FTC) - CPU wins? How?
The problem with your math is, that nvidia is far away from being optimal for GPU mining. If you would compare your CPU with an "older" ATI graphic card like my HD 6870  (about 150$) with max TDP 200W you would surely come to the opposite conclusion. I am mining with that card at ~280 to 300 kh/sec on average. Well I did not measure the extra power usage while it is on 100%, but it cannot be more than 150 Watt, since max usage is 200W. Besides: it runs really steady on 100% GPU load (measured with gpu-z) and drops only sometimes to 99% for a second. Not to mention that nowadays cards can get the double amount of kh/s.

I hope I could help you a bit. If you plan to invest into a new/second graphic card then you should go for ATI.