Take it up in 5 Mhz increments, run a 3D benchmark (like 3D Mark 2001), and see if it makes it through the bench. When it finally crashes. Back it down some (5-10), and see if it will last through a few loops (hour, or so).
Then the real test. Try it at that setting in your favorite games. If it crashes back it down. If not. You're all set.
Every CPU, GPU, and memory is different. Only you can find out how high your individual piece of equipment can go.
