to overclock your CPU, take a look at
this guide
and to overclock your video card, use a program called Rivatuner (universal) or coolbits (Nvidia only)
to use Rivatuner, once it is installed:

from there, move the sliders, test, then apply.
then test it using a program like 3dmark03 or 3dmark05 to make sure it runs okay.
if it begins to artifact, put it down a bit.
once you're happy, check the box to apply settings at startup.