I have a Intel Core i5-2400 @ 3.10GHz and Asus P8Z68-V PRO MB/ CoolMater fin/fan CPU cooler
1.) What is the safest / highest over clock speed?
It has a locked core multiplier as it's a non-K version of the processor so overclocking will be tricky at best, if not impossible. If you manage to do it somehow just keep an eye on your temperatures. That and how much your pc crashes will tell you what's stable.