Overall in the change from the Intel® Core™ 2 Quad to the 1st generation Intel Core processors at the same speed you could expect somewhere around a 15% to 20% performance increase at the same speed. Going from the 1st generation Intel Core processors to the 2nd generation Intel Core processor just yeild another 10% to 15% performance increase at the same speed. So you most likely would see a 25%+ increase in performance with not counting the increase in clock speed of going from the Intel Core 2 Quad Q6600 to the Intel Core i5-2500 or even better the Intel Core i7-2600K. For rendering you should be able to see some big improvement in performance.
Intel Enthusiast Team