Before you go out and buy another heatsink, you might want to consider re-arranging the devices in your case. Make sure the IDE ribbon cables aren't obstructing airflow. You might even want to consider buying rounded cables. (you can make em yourself, but I don't recommend that you do it unless you really know what you're doing and very good with an exacto blade.
Secondly, you can go into your BIOS and set the core voltage yourself to the correct value for your processor. (sometimes the BIOS will set a higher value than normal if set to "auto").
If you leave the cover off of your case, how low does it run?