OK how to over clock them and have differential clocks. Lets say green on top set it to clocks that you know that work. Then select the lower card and max it out as well, riva tuner has options for each card for overclocking as well the fan speed. Once set then load gpu z both cards should have different clocks. A typical non green edition 9800gt that has some what decent cooling should at least get 670 with the shaders linked. As for mem each card is different my top card can reach 1ghz physical while the bottom one can only get 930mhz physical. Temps on both card need to be kept below 70c to insure stability. Power it doesn't take much unless something else is producing a drain on the psu.