i5-3570 has about 10% more processing power than the i5-2500 (directly comparable since they are both 4C/4T CPU's).
Impossible to assign a "value" to that especially when I don't know what graphics card you have and what GAMES you play.
If you want to GUESSTIMATE then use PASSMARK to compare. Look at the SINGLE THREAD performance primarily, but if you can get an i7-2xxx or 3xxx that is compatible with your motherboard for a reasonable price then stick with it.
Whether your motherboard and cooling supports overclock also matters.
My i7-3770K (OC to 4.3GHz) is still a great CPU. I have no plans to upgrade for several years.