For software or games loading speed and booting.
What is your current HDD?
The best improvement for those will be achieved through usage of an SSD.
Games rely more on GPU than proc. You will notice the difference but wil not be optimal and mostly not on the fps on games because your GPU is still the good old GTX260.
Basically E6750+GTX260 can still perform decently, it is better to upgrade your whole system next year (mobo, proc, RAM and GPU) and see what will the best buys be at that time.
Well, as guanyu said, games rely more on the GPU than anything else. However, the difference performance between a Core 2 Duo E6750 and a Core i5 750 is quite large, and games which use more than three cores will be a lot faster. For most applications you will probably see a performance boost, although a faster HDD is the main factor in decreasing bootup time. Some games you would probably be able to see an approximate 30 FPS increase, others 10FPS or less.