Going from Intel® Core™ 2 Duo to a 1st generation Intel Core i3 at the same speed should yield a 15% to 20% performance improvement. Going from 1st generation Intel Core i3 to a 2nd generation Intel Core i3 at the same speed may yield 10% to 15% addition improvement. In the end you most likely would see a total improvement of about 30% between these processors. Also the Intel Core i3-2310M should be better on total battery life.
The Intel Core i7s are going to be running hotter but they are designed for a laptop enviroment so they shouldn't be overheating. They will however have a shorter battery life.
Christian Wood
Intel Enthusiast Team