Maybe 300MHz - 400MHz.
The only way to overclock the i5-2500 is to increase the frequency which basically affects all the components. Overclock too much and certain things may not work or become damaged like the USB ports.
The i5-2500k has an unlocked multiplier which make overclocking much easier. I seems many people are able to OC the i5-2500k to around 4.5GHz.
The i5-2500k also has the faster Intel HD 3000 graphics core vs. the Intel HD 2000 graphics core in the i5-2500. The Intel HD 3000 is basically equal to a Radeon HD 5450.