It is difficult to answer without knowing what your current heat sink is. However, in general heat dissipation through the cooler would be the better upgrade if you're using stock cooling. Heat transfer from the cooler to the air is more important (very generally speaking) than air flow itself. However, both are important variables, so if your cooler is aftermarket you would then want to increase airflow through the case in some way as the next improvement.
I wouldn't consider that too high, but if you want to lower temps your most cost effective way of doing so would be to upgrade the cooler, assuming you have already done the free solution and cleaned all the built up dust from around the current cooler. To be honest unless you plan to go crazy with the overclocking I'm not sure I would spend any money at this point given your relatively low temperatures. Others can add to this if I'm in error.
I'd strongly suggest an aftermarket cooler, however that case wasn't designed with overclocking in mind so it'd def. be a good idea to replace it with a more spacious one with better air flow as the second upgrade.
From my own research I've seen the CoolerMaster 212 EVO being repeatedly mentioned as an excellent performer when on a budget but I am not sure if your current case will fit a tower-type cooler. To know you should measure from the end of the motherboard to the side-panel of the case. You'll need to have 17cm at the very least for a truly marginal clearance (again as my own tests showed me).