If i upgrade my cpu to Q6600 how much fps will i get ?
It depends. However, you should get over 40 fps. With my AMD Phenom II X3 720 and ATI HD 5750, I get close to 50 fps.
As for upgrading your CPU, if you're sticking with Intel, you'll probably want the i7's which is a different socket than the Core 2 Duos. They are faster and better than the Core 2 Duos and will give you the boost you need.
Or, if you really want to save money, you could go to an AMD Phenom II. This again would require an upgrade to your motherboard. If you do go AMD, go with the Phenom IIs. Stay away from the Athlon IIs as those have only two caches as opposed to the three caches that the Phenom IIs have. It definitely does make a difference in performance.
Also, whether you go Intel or AMD, you might have to upgrade your memory as the newer boards require DDR2 or DDR3 memory, unless your current board is running DDR2. Then, you're good to go.