Well you will see a considerable boost in game play but I would wait on buying anything AMD until April 26 they are releasing a new 6 core processor which will most likely drive some prices down.
Now you should tell us a little about the system you have like what motherboard you have how much RAM and what speed is it and tell us your PSU also this will helps us come to a better understanding of what you are working with. Depending on your motherboard overclocking your CPU for the time being can get you a decent performance boost til the hexacore is released a few weeks from now. Also you should look at the Phenom II x4 955 its cheaper and can be overclock by anyone with a simple multiplier adjustment in the bios to match the clock of the 965. But like I said wait til the 26th before you buy anything.