The i5 750 is still a very capable CPU, not quite as good as the 3450 etc but you wouldn't really see a huge difference. Compared to the Q6600, it is quite a lot better (I used to have a Q6600 too, it was a good CPU!).
I agree the Intel® Core™ i5-750 is a nice processor. The move from the Intel Core 2 Quad to a 1st generation Intel Core i5-750 processor would yield around 10% to 15% performance improvement at the same speed. So while you may not be able to keep up with an Intel Core i5-3570K you will have a solid processor.