Modify your 7800GT BIOS to increase GPU voltage by 7%
 

The following guide is a less effective method in that it will yeild a lesser gain in performance than hard-modding, however it is less risky and very easy to return to its orignal state if no gains are realised. It involves changing values in the BIOS of your card to increase the 3D performance mode voltage from 1.4 volts to 1.5 volts an increase of just over 7%. You should notice a small gain in maximum stable overclock, mine now auto detects at around 500/1150 a gain of around 30mhz.