You simply can not match GPUs with CPUs, it's just not going to work.
The hash rate on a decent nVidia card is around 20m/hash. With a decent AMD GPU you should be able to get about ten times as much.
As such, mining isn't really profitable without an AMD GPU or preferably an FGPA or even better, an Asic.
With an AMD 6950 you can get 300m/hash, I believe.
CPU has rates are in the order of k/hash. Simply incomparable.
(Somebody pull me up if I'm completely wrong, please. Lol.)
By the way, I'm pulling 32m/hash with my nVidia 630M 1GB and Integrated Graphics and I'm only just breaking even with the power consumption at 0.27USD per kwh.
So I'm basically converting my power bill into BitCoins at the moment.. lol
This agree completly