G80 and R600 have different architecture
This website explains to you the difference between the shader power in G80 and R600
Both AMD and Nvidia like to throw around peak FLOPS numbers when talking about their chips. Mercifully, they both seem to have agreed to count programmable operations from the shader core, bracketing out fixed-function units for graphics-only operations. Nvidia has cited a peak FLOPS capacity for the GeForce 8800 GTX of 518.4 GFLOPS. The G80 can co-issue one MAD and one MUL instruction per clock to each of its 128 scalar SPs. That's three operations (multiply-add and multiply) per cycle at 1.35GHz, or 518.4 GFLOPS. However, the guys at B3D have shown that that extra MUL is not always available, which makes counting it questionable. If you simply count the MAD, you get a peak of 345.6 GFLOPS for G80.
By comparison, the R600's 320 stream processors running at 742MHz give it a peak capacity of 475 GFLOPS. Mike Houston, the GPGPU guru from Stanford, told us he had achieved an observed compute throughput of 470 GFLOPS on R600 with "just a giant MAD kernel." So R600 seems capable of hitting something very near its peak throughput in the right situation. What happens in graphics and games, of course, may vary quite a bit from that.
As you see, 8800GTX have either 518.4 GFLOPS or 345.6 GFLOPS (depending on how you count them)
