depends on your hardware, the amount you overclock it, and in what game
since mild overclock (few 100Mhz), will give almost no noticeable performance increase in most games
where as upgrading to a bigger GPU would give a huge performance increase
if you do a solid overclock 600+Mhz you will start to notice the performance increase, and in some games it will be more noticeable than others, depends on how CPU heavy the game is
but again upgrading to a stronger GPU will almost likely yield the bigger performance increase you will notice
now if you already have the "top" GPU, and thus cant upgrade, and your only performance increase would be overclocking, it depends on what game, and how much you overclock
100Mhz overclock, you wont notice...