IF changing the GPU speed has no effect on the FPS then the most likely cause is a bottleneck elsewhere !! (ie. your CPU, Memory,HDD) the GPU can only operate as fast as it is given info to process so a slowdown elsewhere will cause it to wait for the info it needs. and if the speed does not change with the OC then that means it is already not being used to capacity so the extra speed is just adding extra idle time not performance.
This means you have a bottleneck in your computer. When you overclock your Graphics Processing Unit, that like changing to a faster car. However if the road still only lets you drive the same speed, you car can't go faster. So your Central Processing Unit, your Random Access Memory, or perhaps something else is bottlenecking you.
That Asrock is junk you need a P45 chipset to get full potential out of that CPU that AsRock is way overpriced because it has DDR3 RAM which isnt even really supported by Core 2 processors and gains you nothing really in performance over DDR2 RAM.
If you were to get a new motherboard so you could overclock you want to look for a motherboard with a P45 chipset like this.