I wouldn't pay more than double the price of a i7 920 for an i7 940! Thats madness.
They're the same chip apart from the stock running speeds, 2660Mhz & 2926Mhz respectively, and the i7 920 will over-clock to at least 3500Mhz at stock volts! Faster than the 940 @ stock.
So unless you hate the words 'Over' and 'Clocking' go for the i7 920, OC it and spend the extra $300 on something else.