How much can you overclock safely? The answer is that you cannot overclock safely, as soon as you increase your product beyond stock speeds you void the warranty, decrease the lifespan, and increase the risk of instability, file corruption, thermal damage, and BSODs.
That being said, the chances of the above happening are slim to none if you know what you are doing.
As far as your maximum overclock possible, that is impossible to gauge and can only be determined by you through overclocking. It varies based on your hardware configurations and how your chip responds.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w