n00b overclocking question

[Ezekiell]

Hi,
I got recently a 965BE C3, 4GB of G.skill 12800 Eco and a Gigabyte 890GPA-UD3H mobo. I'm currently running the ram @ 1333mhz 6-7-6-18 1t @ 1.5v with a 5.33 mem multiplier and CPU freq @ 250 ( I'm doing this because the board will only boot otherwise with ram @ 1333mhz CL9 or 1600mhz @ CL8)

I'm currently at 3875mhz @ 1.45v (not sure if it's 100% stable yet, only did 1H OCCT) and NB freq @ 2250mhz, stock volt.

When you increase the NB freq (my goal is 2.5ghz), which voltage do you need to increase for stability? Is it the NB voltage or the CPU-NB voltage? And what is the max safe volt on air

thanks