TechSpot investigates whether overclocking your graphics card is really worth it, you can read the report over here. The conclusion is it can provide performance gains of almost up to 15%, but the site notes that AMD and NVIDIA are being extra careful about how they build their GPU series to ensure that most cards can't be overclocked to replace the model above it.
Overclocking plays a vastly different role in the computer industry today than it did 10 years ago, a time when overclockers were considered outlaws by manufacturers. Back then even mentioning overclocking could void your warranty and industry leaders like Intel were working to eliminate it all together.
In contrast, nowadays processor and graphics cards manufacturers have embraced the practice, touting high 'overclockability' as a feature and in the process using it to sell enthusiast oriented products at a premium.