The i7-920 is still a good cpu clocked at 2.66 stock.
The i7-930 is clocked at one notch higher at 2.8. , a 8% better result.
In most cases at stock, you will not be able to tell the difference.
Other than the possible clock rate, there is no real difference.
If you overclock conservatively, 3.2 or 3.3 is easy on either chip, making a price premium for a 930 not worth as much. It is possible that a 930 will overclock a bit higher than a 920, but nothing is guaranteed since it depends on the particular chip sample and dtepping.
In August, the i7-950 3.03 price will be reduced to 930 prices. If you will not overclock, you might wait. If you will overclock look for a good deal on a 920.