How high could I raise it without changing the voltage or endangering my cpu?
There is no way to tell that information unless you are God himself. That primarily pertains to the 'without changing voltage' part, but as for the 'endangering CPU', well we CAN give you some info to that.
Don't go too far above 1.6v especially if you dont have good cooling. Even if you have really good cooling, I personally probably wouldn't go above 1.65 and even then that's pushing it a little bit. This wasn't a problem in the Athlon-XP days, but with the memory controller being on-die, raising the voltage too high can fry out the memory controller. Don't ask me how often that occurs though because I don't know.
Also, although it's cool that you're diving into overclocking, I wouldn't really consider it "My first overclock" when all you did was bump it up 10FSB for 5 mins then knock it back down.
When you claim an 'overclock' then it should be something you've raised, tested (prime95, gaming etc..) and is stable. THAT's an overclock. Anyone can knock up the FSB temporarily.