Overclocking GeForce 8800 series graphics cards involves some uncommon peculiarities we want to discuss here. As you perhaps remember, early products from the GeForce 7 series on the 0.11-micron G70 core could only increase the frequency of the ROPs and pixel processors with a step of 27MHz. Nvidia then returned to the standard frequency step of 1MHz in G71-based cards, but the GeForce 8 series has a discrete frequency step, again.
Shader processors in the G80 chip are clocked at a higher frequency than the rest of the GPU subunits. The frequency ratio is about 2.3 to 1. Although the basic core frequency can be changed with a step smaller than 27MHz, the frequency of shader processors is always changed with a step of 54MHz (2x27MHz). This raises more problems at overclocking because all the overclocking tools deal with the basic frequency rather than with the shader domain clock rate. There is a simple formula to calculate the frequency of the GeForce 8800’s streamed processors during overclocking with a high enough precision:
OC shader clk = Default shader clk / Default core clk * OC core clk,
where OC shader clk is the (approximate) resulting frequency, Default shader clk is the initial shader processor frequency, Default core clk is the initial core frequency, and OC core clk is the frequency of the overclocked core.
Let’s now see how the MSI NX8800GTS-T2D640E-HD-OC behaves when you try to overclock it with RivaTuner2 FR. This program can show the real frequencies of different areas, or domains, of the G80 GPU. The MSI card has the same GPU frequencies (576/1350MHz) as the GeForce 8800 GTX, so the information below applies to the flagship card as well. We were increasing the basic GPU frequency in 5MHz steps. This is a small enough step, and it is not a divisor of 27MHz.
So our practical check shows that the basic core frequency can indeed be changed with a variable step: 9, 18 or 27MHz. We could not spot any regularity in the change of the step, though. The frequency of shader processors is always changed in 54MHz steps. That’s why some frequencies of the main domain of the G80 chip prove to be practically useless at overclocking. Using them only heats the GPU up. There is no sense in overclocking the main core frequency to 621MHz, for example, because the shader block will still be clocked at 1458MHz. So, you should overclock your GeForce 8800 with some caution, using the formula above and looking up the monitoring data of RivaTuner or some other similar program.
We had not expected anything exceptional from the pre-overclocked NX8800GTS, but the card surprised us with its high GPU frequency potential. We managed to speed the GPU up from its default 576/1350MHz to 675/1566MHz and the NX8800GTS passed a few test cycles in 3DMark06 without additional cooling. The GPU temperature was 70°C as reported by RivaTuner.
We were less lucky with the memory chips whose default frequency of 850 (1700) MHz had already been higher than their rating frequency of 800 (1600) MHz. We stopped at 900 (1800) MHz. At higher memory frequencies the card would hang up or issue a driver error message.
Thus, the graphics card has a good GPU frequency potential, but its memory chips are not that fast. The level of the GeForce 8800 GTX is already an achievement for them. Coupled with the 320-bit memory bus, this ensures a considerable advantage over the Radeon X1950 XTX in terms of memory bandwidth: 72GB/s against 64GB/s. The overclocking results will vary depending on the particular sample of the MSI NX8800GTS OC Edition and your using or not using additional means like volt-modding or installing a water cooling system.