If you talking about a copper shim, don't waist your money, alot of people I've talked too, and one particular poster here in the last couple of days got a shim that was just a tad too tall and wasn't getting correct core pressure from the heatsink. Resulting in a 10C degree increase. He took off the shim and viola, he was 10c degrees cooler.
If you know how to properly install a heatsink, shims are not needed, and really aren't that big of a deal. Mainly I beleive they are a direct reflextion of the hardware sites wanting to cash in on the 'crushed core" situations. Instead of just explaining how to do it correctly.
<b>"These are my thoughts, your mileage may vary."