if i get a g3258, do you guys think i can OC to 4.0 GHz with the stock cooler?
Yeah, you can although you should probably do cable management and ensure you have good airflow.
Source: I did it
idk man this thing can hardly cool itself. I was getting above 65 on stock with my i5.
You'd be surprised, that CPU overclocks exceedingly well. Also 65 isn't that bad considering that the recommended safe temperature is under 80.
I normally get 50-65C on the g3258 on stock cooling when OC'ed as mentioned before.