The difference, however, is that you'd need a newer motherboard to go with the i5-4670. Additionally, the performance gain in Intel chips from one generation to the next, for the same clock speed, isn't much. It's there, but it's incremental.
If you can get an i7-2600, that would be the best bang for the buck, as it should be a drop in replacement for your i5-2500.
For games, generally the hyper-threading of the i7-2600 won't give you much benefit.
Here's a more pertinent question, though. Is your current system falling short in any way? It's a little odd that the current i5 is running at 99% and the i7 at 70%, considering that there's only a 100MHz speed difference.