It's not a matter of safe; it's a matter of stable. Unstable is not unsafe, it just leads to a lot of crashes and needing to reboot. Overvolting can be harmful.http://www.cpu-world.com/CPUs/Pentium_Dual-Core/Intel-Pentium%20Dual-Core%20E2180%20HH80557PG0411M%20(BX80557E2180%20-%20BXC80557E2180).html
I would not push the voltage much higher than stock.
The only way to know the overclockability of your CPU is to try. Start off overclocking until you reach unstable levels without overvolting. Then lower the ram multiplier and try overclocking more. When you reach a limit (ie a stable overclock) then you can try going higher by increasing the Vcore while increasing the frequency.
At a guess, I doubt you could get to 3GHz. Maybe 2.5.
What settings do you play at now and what resolution? From a small overclock on that old CPU, you cannot expect much of a performance increase.