Well you can somewhat OC that CPU as its almost the same as a e7500 which is slightly ocd from your CPU.
I would suggest getting a better GPU to power your system and play most games. What is your actual budget for gaming/GPU? PLs try to sell that GT240, its really not worth keeping on your system which has alot of potential. Just get at the very least a GTS250 or a, 5770, or a 4870. These GPU's are still very good and can play most games at their high settings.
You will thank me for this as you will immediatly see and feel the difference.
That 5770 is a good choice and of course overclocking it will stretch it's life even longer. In a year or two you can add another 5770 and run them in crossfire. Anyway, your motherboard has some overclocking options so just read an LGA775 overclocking guide and you should be able to get a decent overclock out of your CPU. When overclocking the mixed RAM will likely be the limiting factor. I recommend you get a good tower cooler for your CPU and you may also want to get a small fan to attach to the north bridge.