Quite a few games tend to do better with three to four cores nowadays,but,it really depends on what games you are playing. Some won't show much improvement.
A Q6600 should not be more than £60 to £70 secondhand and even my high VID example hit 3GHZ with a SFF PC CPU cooler.
However,what motherboard do you have as this can limit your overclock and what PSU do you have too??
TBH,with price of DDR3 being so low,you could always get a Core i3 2100,8GB of DDR3 and a Z68 based motherboard for around £200.