It's very close, and the architecture is the same. An i7-930 will perform basically exactly the same as an i7-975 at the same clockspeed (there is some slight variation due to different base clocks at the same core speed, as well as different uncore multipliers, but that's fairly subtle). The main reason to get a 975 is because it should overclock significantly higher than the i7-930 if you were pushing it.
Is the i7 975 an overclocked version of the i7 930?
If I overclock the 930 successfully to 3.33 will it have the same performance?
Or is the 930 only the same as the 960
The 975 has a higher multiplier than the 930, but the same base clock of 133.
If you overclock the 930 you will raise the BCLK from 133 to 160, giving you essentially the same performance.
No overclock is guaranteed on either cpu, but experience has shown that every 930 I know of can reach 3.33 easily.
If you are looking at overclocks of over 4.0, then it may be more likely that a 975 will give you a better binned chip.
But, I don't think anything greater than a 930 is worth it if only a conservative overclock is sufficient.