You really need a quad core. The game is limited by the CPU. In fact, your 5770 will get bottle necked by the CPU. The main reason is because when they ported GTA IV to the PC from the consoles, they didn't do a good job of optimizing it. In fact, don't expect the game to look really good, even on the max settings because of this being ported from consoles.
I had an ATI HD 3870 on an AMD64 X2 4200+ and I was lucky to get 25 FPS on the lowest settings.
I upgraded my machine to an AMD Phenom II X3 720 Black Edition and immediately, my FPS went to around 39 on the highest settings with my 3870.
Today, my 5750 came in, but since I'm at work, I can't say how much better it will be, but definitely, dual core CPUs are not enough for this game.
What he said. If your Core2Duo is an E7400 or or an E6+XXX, you should be alright with bottlenecking. However, a good overclock to 3.4 ghz will help to solve your problems, though ultimately a Q6600 would be much better..
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.