Safety of overclocking is dependent on CPU voltage settings. While small increase in working voltage will help stabilize system for higher speeds, heat dissipation will become problem. Moreover, each processor has reference maximum voltage. The voltage number is limit when overclocking CPU. Your motherboard will help you lower voltage requirements if it has good voltage regulators, which usually are found in pricier motherboards, hence allow for stable overclocking environment.
Watercooling is a good start for any overclocking. I strongly recommend it for both CPU and surprisingly GPU! While not easy to find mounting methods for closed watercoolers for GPU, temperature-wise it is simplest and not expensive way to overclock GPU...