As great as the 920 is and like maziar said the 930 is good too. The 920 is getting discontinued to move way for the new chips intel is hoping to release this month. If I were you, i'd actually wait till the 950 drops pricing, which would lower the 930's as well.
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.
But will the 950 going down to the 930s prices make things (like the 930 and 920) cheaper or are the discontinuing those?
I think Intel is clearing out inventory of 45nm quads in anticipation of Sandy bridge 32nm cpu's coming out this fall. If your time frame is within the month, then get the cheapest 9xx cpu you can and give it a conservative overclock. If you can wait until the end of the year then look to the new line of cpu's. They will all strart out clocked >3.0 and will probably perform better clock for clock because of the revised architecture.
As to prices, I expect not much drop on the 920 or 930. Since all three chips can be overclocked to about the same limit, their value will remain the same, giving a slight premium to the faster ones.
Though with the revised architecture you still maybe able to OC higher but until prices are announced for the 32nm cpu's we'll see if the price is worth it. For instance the 32nm's might oc more and easier, but in my opinion once you get to about 4ghz, there really isn't a point for basic or gaming use. But on the otherhand, with the 32nm design it allows for lower temps with higher clocks. Though, after 4.3 or 4.4 ghz (if you get it there) The cooler you use will determine it. Mostly because at 4.3, if i'm correct i7's get VERY hot. And will get hot even with the 32nm.
If I recall properly, each time Intel changed architecture, the performance of the cpu improved by 10-15% on a clock for clock basis.
No doubt, they will try to sell the new chips for what the market will bear. With 32nm, the yields per wafer go up quite a bit, making their cost per chip go down. As a result, there will be headroom to lower prices if they want more volume, or if AMD gets too aggressive on pricing.