Graphics Card Overclocking Guide
First step in overclocking a graphics card is to pick a fan speed that you are comfortable with the noise level. GPU stability is highly dependant on the maximum temperature that the card reaches while gaming so picking a fan speed other than auto really helps to keep the temperature stable.
If you were to graph the temperature and maximum stable clock speed it would look something like this as long as the voltage remained the same. Increased voltage will cause the maximum stable frequency line to move upwards on the temperature axis.
Now that the fan speed is set it is time to run the first stability test and find out the maximum temperature. I prefer rthdribl from http://www.mediafire.com/file/wdhjvrpuzellz59/Video%20Card%20Stability%20Test.zip for the stability test. It is more representative of a real game than something like Furmark but if you want absolutely perfect stability go with Furmark.
With my fan speed set to 100(I wear headphones so the noise doesn't bother me.) and clock frequencies at stock(675/1675/1100) my maximum temperature is 67C.
First clock I'll start working on is the core which is linked to the shader clock on nVidia cards. On ATI cards there is not a shader clock. You will probably want to test in increments of about 25Mhz. I know my card quite well so I did rather large increments at first.
After moving core to 750Mhz my maximum temperature is 70C but the card remains stable so I'll continue to increase it.
Now it is set to 840Mhz and it reaches 69C before the drivers crash.
This leaves me with 3 options or a combo of them. A, decrease the temperature.(Not that easy since I'm on 100 percent fan.) B, Decrease the clock speed by a small increment until it becomes stable. C, Increase the voltage. I'll be using primarily option C with a little bit of A.
Now I've increased the voltage from 1.225 to 1.300. At 840Mhz it will reach 78C and still maintain stability.
At 860Mhz with the voltage increase it crashed at 73C. Time to add more cooling by taking the side panel off and a little more voltage.
With the side panel off and 1.332V it holds steady at 72C and no crashes.
880Mhz crashed at approximately 66C with the 1.332V. Time for more voltage.
At 1.408V and 880Mhz temperatures increase quickly to 78C causing it to crash.
Core overclocking conclusion:
Adding voltage will continue to put off the temperature limit but eventually the added heat from the higher voltage will cause it to reach the increased temperature limit.
With the side panel on and a minor voltage increase 840Mhz is the limit of my card. With the side panel removed 860Mhz is possible and 880Mhz is possible for short benchmarks. Not bad for a card that starts out at 675Mhz.
Memory overclocking is mostly the same as core overclocking. Leave the core overclock at the highest stable setting and start increasing the memory slider and allowing the card to reach maximum temperature between memory frequency increases.
On an ATI 5K or 6K series card it is necessary to run a scored benchmark such as 3D Mark 11 between each memory speed increase as they use ECC to prevent crashes. Instead of crashing the performance will be degraded.