WARNING:
To all the noobs screaming "OMGz ONLI 64 SHADERZ?! NV HAS 128 LOL! 8600 WILL HAV 64! OMGZ LOL PWNT!"
Realize it said 64 vec4 shaders. Vec4 shaders are a more advanced shader design than the design in G80, which uses 128 scalar shaders.
Scalar shaders can do one shader operation per cycle, vec4 can do a maximum of 4 operations.
This means that theoretically the R600 could be as powerful as having 256 G80 shaders --BUT -- not necessarily. The problem for ATI is that scalar shaders are so simple they are always at 100% usage (meaning no wasted power in the G80), but their R600's vec4 shaders are more complex -- 100% usage is very hard to achieve, they'd be lucky to get 80%.
Expect both cards to be within 20% of each other, maybe more in some, less in others...
My money is on R600 for DX10 performance, maybe a tie in DX9. This has yet to be seen however..
So, remembering that 64vec 4 = ~256 scalar shaders (NV = 128 scalar), plus the fact that R600 is 512-bit external memory bus w/ gigabit (1024bit) internal ringbus (NV = 384-bit internal/external), plus higher core and mem clocks at rumoured 800/1100 (NV = 575/900)... don't count ATI/AMD out of the game yet...