From DD-WRT Wiki
OVERCLOCKING CAN BRICK OR PHYSICALY DESTROY YOUR ROUTER!
USING THE DD-WRT OVERCLOCKING FEATURE IS AT YOUR OWN RISK!
WE GIVE NO SUPPORT ON IT OR ON BRICKED AND DAMAGED ROUTERS!
With DD-WRT you have to chance to speed up things a bit ;-)
Broadcom
Atheros
AR2315/AR2317