shims do nothing. stock heatsinks just barely keep the processor running, and if your motherboard temp is over 30C, a stock heatsink isn't going to cut it.
first of all, you need some air flowing in and out of your case. Get at least one fan for intake (usually put in the front), and one for exhaust (typically in the rear). The point is to suck cold air in the case, and blow the hot stuff out. Once you get some air flowing through the case, your ambient temp should drop, and your cpu temp along with it.
for $10-40 you can get a new heatsink that will help immensely. check out
www.sidewindercomputers.com or
www.nexfan.com