Overclocking Problem.

Hey I am trying to overclock my AMD spemron 2800+ but it doesnt work. The problem is when I go to the bios and go to advanced chipset features there is a option called "system performance" where i can choose between optimal/aggressive/turbo/expert. I want to put it on aggressive but when i do put it on aggressive and restart the pc, the screen stays black. Why is this?

You may have to increase the voltages a bit for it to post if there isn't a Auto voltage adjustment or if you have increased the clocks you may want to reset them to default values for the performance options to work.