Join AnandTech in looking at 4 different manufacturers overclocked 8800 GTS and GTXs although only 2 offered the lesser 640MB versions as well as the full 768.  Not every manufacturer has set the clock speeds to the same frequency or even the same ratios.  As well the voltage and temps differ between the cards.  Take a look at the impact that memory and speed has on these otherwise identical cards.
“Our roundup features four different manufacturers that currently provide factory overclocked parts. While we sent out requests for cards to various manufacturers, only four responded and two of these provided both GTX and 640MB GTS hardware. Overclocked 320MB GTS cards have also come out, but we are planning a separate roundup for these (both factory overclocked and stock) in the near future.”

Here are some more Graphics Card articles from around the web:

Click Here to go to Video Cards   Graphics Cards