If there wasn't enough voltage getting to the RAM, the machine might not "POST", (Power On Self Test), or you might get errors.
Some high performance RAM will run at a lower speed a the lower voltage, then run at full rated speed a a higher voltage. DDR2 1066Mhz seem to be the place where this is most prevalent. Most of these DIMMs run at 800Mhz (effective) until the voltage is stepped up to > 2.1 volts.
The timing of the RAM and the speed of the RAM are two entirely different attributes, don't confuse them.
Nvidia settings might need software installed to function (.), (!), or (?) Pick the punctuation mark you feel most comfortable with.
I doubt that Nvidia supplies the BIOS itself, and that's where you should be looking to adjust RAM voltage.
I'm >> guessing<< that Nvidia has similar software to Gigabytes "Easy Tune", which allows you to adjust system performance from within Windows.
The RAM is running at the correct speed. My suggestion is to run "Memtest86", and see if any errors occur. If not, leave the voltage setting alone.
If errors do occur, then insufficient RAM voltage could be the cause.
http://www.memtest.org/ Download memtest from here;
http://www.memtest.org/ (ISO zip) use Nero or similar to burn to CD. This is an.iso "image" file and must be burned as such, don't try to copy and paste it to a CD.
@CCT; That makes sense doesn't it, after all you have two separate pipelines of memory running simultaneously. But why the heck do they call it "PC6400"......?