The i7-920 is the same chip as the i7-975. The intrinsic capabilities are the same. Some samples are easier to overclock than others. I consider a safe overclock to be 3.33 or so, the same as the base speed of the 975. As to the cooler, if your temps are in the 70c range, chances are that the cooler was not mounted well.
Push pin coolers can be tricky to install.
A bad installation can lead to higher temperatures, and even cpu throttling.
With the pc powered down, gently rock the cooler to see if it is on solid, or if it wobbles a bit.
Push pin coolers are best installed while the motherboard is outside of the case.
You need to be able to look at the back of the board to verify that
all 4 pins are completely through and locked.
Play with the pins on the cooler first, so you can see exactly how they work.
Read the instructions that came with your retail cpu.
When pushing down on the pins, do a diagonal pair first.
If you don't, it is hard to get the last pin in.
Don't forget to clean the parts and reapply fresh thermal compound every time.
Don't try to reuse the TIM.
Rubbing alcohol is OK as a cleaner.
I use a paper coffee filter to clean with because it is lint free.
Any name brand TIM should be OK(as-5, Mx-2, etc.)
When applying the TIM, don't use too much, because it can act as an insulator.
Don't apply too little, either, because it won't spread and fill the microscopic
imperfections in the surfaces. A dollop about the size of a grain
of rice should be about right.
Still, I would suggest you invest in an oem cooler with a 120mm fan. Your cpu will run cooler, quieter, and overclock easier.