The only overclocking done on my PC is the graphics card, and the factory did that
http://www.bfgtech.com/7600GS_512.html
that's the one
oh.. I suppose the addition of a couple of artic cooling fans could be seen as a "mod"
http://www.arctic-cooling.com/cpu2.php?idx=79&disc=
and one of their case fans
I think really with the price of chips falling and kit getting cheaper, there is not as much need for overclocking, but having said that there seems to be no shortage of people willing to spend loads of money on a box that only a few people will see