I did go from a QX9650 to a i7 2600k, though mostly because my old PC bit the dust [motherboard issues with my 790i and a dead GPU]. There is a performance increase, but personally, you should at least wait for Ivy Bridge, if not Haswell to release. You still have plenty of processing power.
At equivalent clock rates, there is a small improvement from say the core2quad to an i5-750, then another improvement from that to the sandybeach processor. Of course there is another adavantage because you could get the new processor to 10 or 20% higher clock rate. So overall you could improve your CPU performance by 50% or more. However Toms tests have usually shown that once your CPU gets to a certain point then the GPU system becomes the limiting factor.
Someone recently posted an overclock of their 2500k to about 4.5GHz, along with their SuperPi 32M times. I'm sitting at an E8500 at 3.8GHz and the 2500k almost cut my times in half.
Now if you were running video encoding or something like that, it would be worth the upgrade.
Yeah and sell me your 9650 when you do, when you have as many kids as I do You can not afford to get a totaly new setup.
I've promised the QX9650 to a friend (who already has a mobo and memory) If you're serious about building another PC, I have a Q9650 (only used for 3 months before I was able to grab my current cpu) and the mobo/memory (as shown in my current system specs) if you would like it.