I'd agree with SR-71. For now you have around $60 or so invested in the g3258. An i5 would be a nice upgrade for games without going crazy or full top end. The i3 will be a slight upgrade but then you'll probably be left still wanting a bit more performance and end up looking toward an i5.
By then you'd have spend around $120 or so on the i3 and another $180 on the i5 and then you'll have $360 tied up in a system still running an i5. Or at the very least around $180 tied up in cpu's and still running an i3. I would save up just a bit more and skip the i3, invest in a cpu that will last you several years with good performance and be done with it.