All i7 -9xx chips are the same. Main difference is basically their base clocks/multipliers unlocked or not.
An i7*965 is no better than a i7-920, all is the clocks 2.66ghz vs 3.2ghz
I have mine running stable at 3.43ghz at stock fan and NOTHING altered.
Its just that easy and i know plenty of people with 920s 860s i5-750s that can do the same in just a few clicks. And with that said, easily negates the higher prices of the higher clocked brethrens.
And if you think higher price = better, its somewhat true as the higher quality chips tend to go into the higher clocked ones such as the EE editions and the 2.93ghz one and the xeons, but basically, all the chips are the same.
Historically, I have always bought the 2nd fastest Intel chip but the 920's overclockability just made it too attractive. I really wanted to try my hand at "pushing" the OC envelope this time around so that I didn't want to risk frying a $1000 CPU.
While you can certainly OC a 920 past 965/975 stock levels, it's also certain that the 965 / 975 will OC far past a 920. The 975's been OC'd over 6.1 Ghz....965 at 5.7 and 920 at 5.4
As to whether to buy one, it depends on your goals....I think this paragraph puts it well:
Intel's Extreme Edition processors are not for everybody. Economically, they just don't make much sense for mainstream users. These extravagant chips were designed for smaller segment of the market where the consumers who buy them tend to push their hardware a lot harder than most folks. Here, workstation professionals, overclockers, benchmark freaks, and extreme enthusiasts spend as much time tweaking system settings for optimal performance as they do operating their computers. Besides the small speed bump that the 975 features at its stock settings, the main advantage it has over its more affordable siblings is an unlocked multiplier which provides coveted flexibility to overclockers as they push core speeds to new heights. If overclocking is an art, using an Extreme Edition processor is like painting with the finest tools on an immaculate canvas.....The price premium is a major hurdle for those who want the best, but that seems to be the case with most high end products.