The Athlon XPs use an organic packaging, which IMHO is much more resistant to cracking and chipping than the Thunderbird's ceramic packaging was. For a new Athlon I wouldn't even consider a shim unless you are using an extremely heavy heatsink and want think the pads may not be enough to support the weight of the heatsink. If you choose to do this, make sure you do NOT get a conductive shim, as there is a remote possibility of shorting stuff out.