Well normally I'd suggest an SSD as that would be the most noticeable upgrade for you but you already have an M4. The only option would be Overclocking which is EXTREMELY easy especially with your CPU/MOBO/Cooler combo! You should get an easy 4.5 Ghz OC just by hopping into the bios and changing the Multiplier to x45! Otherwise there's not much you can upgrade that would make a difference with those games. Possibly multiple monitors would be an upgrade unless you're more into a larger single monitor. You're system seems to be overkill for those games as it is! \
Not that there's anything wrong with that

, you're pretty much set for a while no matter what you choose to do whether you OC or not.
Nick