The stock speed for the GTS 640mb is: 500 GPU and1600 memory, for the 512mb GTS they are 650 GPU and 1944 memory. The GTS 640mb has the advantage of 320 bit memory vs. 256 bit but it is overwhelmed by 512MB's extra pipelines and speed. In an earlier HardOCP review, they had the Crysis demo recommended setttings at 1280x1024 for the 8800 GTS 640mb and 1600x1200 for the 8800 GT. The 8800 GTS 512mb is a bit faster then the 8800 GT.