All these things are a trade-off.
If you stick with the stock cooler, you will have a very good chance of achieving the overclock you are aiming for. The temperature will be a bit high (for the whole system) and the niose levels may prove unacceptable.
The AC7 is inexpensive, will certainly drop your CPU and system temperatures by several degrees and will be quieter than the stock cooler.
Any of the more expensive coolers mentioned will give you a few degrees more cooling and be significantly quieter than the AC7.
It is very much the case that you get what you pay for. To my way of thinking the AC7 was the best trade off between price and performance and I am happy with my choice. Your needs may be different, but from what you have already said I suspect that it may be the right choice for you.
Another point to consider is that the AC7 can be installed in-situ while the others have backplates which mean that the motherboard has to be out of the case to fit them. If you are going to invest in one of the more pricy coolers, it is better to get it from the outset as changing to it later will be a bit of a pain.