depending on what motherboard you have, you may have some options built-in. My Asus A8N-E has Cool n Quiet, which underclocks the cpu when there is no load and brings it back up to full speed whenever you need it automatically. I use this all the time, it's a great way to keep temps down. They also have a dynamic overclocking options called NOS, i think, and it does the opposite: overclocks your cpu by 2, 5, or someother percentage whenever it's under full load. Using both of these in tandem may be what you're looking for, but I don't know what mobo you have.