it is done using nvflash and nvbios editor. i first did an extensive software OC test on my GPU, get the perfect balance between speed and stability, only then i start tweaking the BIOS based on the data i gathered from the software OC test. as u might already have known, each hardware component is different from one another. 650MHz core 550MHz memory is stable for me and i can go higher if i wish but for stability sake, this is what im sticking to. i've even seen 1520 wont get pass 500MHz for GPU memory (and my previous 1520 GPU memory wont get pass 510MHz). so it's entirely system dependant. im sure u get the idea. just a quick side note, i have been running my system for almost 2 years now at these "extreme" speed, and have never encountered any problems before. so make sure u do ur homework right before taking the dive.

a word of caution...
i wouldnt recommend this method unless you are absolutely sure what you are doing and you are a very advanced user. altering and flashing BIOS can be very risky and would cause permenant damage to your hardware IF not done properly. so please proceed with extreme caution.