Increasing the core voltage can be dangerous, but again, if you follow a guide from a trusted source and are prepared for the consequences of overclocking your machine, then there shouldn't be a problem.

In many cases you will actually need to increase the core voltage - After all, to run faster, the CPU will require more power - if it is already borderline at 1.5V, it will naturally require a higher core voltage if you were to overclock it.

However, increasing anything concerned with the CPU - Clock speed, multiplier, core voltage and so forth - should be done in small increments, one at a time. After each increase, you should test your PC thoroughly, especially under high load - When you start to have problems, simply roll back to the previous working settings and you have in effect the maximum that you can safely go to.

As for overclocking - you really should leave it unless you know what you are doing and are aware of the consequences. In truth, you will reduce the lifespan of your components, and the extent will depend on the level of overclocking. You can also quite easily go too far and end up with fried CPU - Though most machines these days have failsafes, such as shutting down the machine when a specified CPU temperature is reached, it shouldn't be relied upon - This really is at your own risk.