The question is milage vs money.
.. Sure the QX975 will be a great boost to CPU power - BUT that's it - How long tell you find out your missing out on the other features.
.. Main reason I went from a I5-750 ->I5-2500k was NOT the CPU but because of the MB. Sata III for that future SSD, USB3 for faster external HDD and for USB3 thumbdrives. Pcie 3.0 (not needed today, or tomarrow - how about a year from now???
If you do any video encoding then you would be missing out on Quicksync (Requires SB CPU + Z68 MB.
Sure you can add a pcie card (maybe), but the sata III is 3rd party controler and not as good as Intel Sata III MB controller. Also # of devices limited by PCI-e lanes which may cap your new devices to sub-par performance.
If Gaming is your only use, then the Q975 will satisfy you for the near future, but a year from know, if you end up finding the other MB features compelling; then that $$$$ may not have been cost effective.
Basically, you don't want to throw away money on an old system.
I would say get a coolermaster Hyper 212+ for your CPU and overclock it. It should help tide you over until you get a new system. Its around 40 bucks. Your 2.66Ghz should easily do 3.0Ghz or even a little more with a better cooler.
The change from the Intel® Core™ 2 Quad to the 1st generation Intel Core processor give a 15 to 20% performance improvement at the save clock speed. The change from the 1st generation Intel Core processors to the 2nd Intel Core processors of which the Intel Core i5-2500K is a member would have given an additional 10% to 15% performance increase at the same clock speed. In the end you would see something around a 30% total improvement in performance. Now understand this wouldnt all translate into a 30% performance increase in a game since most of the gaming performance is based off the video card. This also doesnt cover any overclocking.