How to OC i5 750 to 3.4 ?

Each overclock is unique due to the variability in the individual components. Just because someone might achieve something with exactly the same components that you have doesn't guarantee that you will achieve that mark. Having said this, a goal of 3.4MHz is probably very realizable if you have the proper cooling solution (read, aftermarket).

Yes, changing the baseclock to 170 will result in a CPU speed of 3.4MHz. This will downclock all the other components as well.